iris: Only copy existing data into staging images with PIPE_MAP_READ

iris: Only copy existing data into staging images with PIPE_MAP_READ

When performing transfer maps on images that require staging buffers
(say, for presenting a linear view of tiled memory), we were reading
the existing contents of the buffer into the staging resource on map
unless PIPE_MAP_DISCARD_RANGE was set.

The thinking was to support partial writes.  If you map a subrectangle
of an image, but then only write selective pixels - should it preserve
the existing contents of the mapped region?  I believed that it should,
unless you pass PIPE_MAP_DISCARD_RANGE to explicitly say that that it's
okay to invalidate the destination region.

However, that does not appear to be the interpretation favored by other
Mesa developers (in particular Michel Dänzer and Marek Olšák).  The
radeonsi driver does not do this readback from the destination region
to the staging buffer unless you pass PIPE_MAP_READ.  If you want to
do a partial write and preserve contents, you need to pass both flags:
(PIPE_MAP_READ | PIPE_MAP_WRITE).  Passing READ is expected to come
with an associated cost.

OpenGL defines GL_MAP_INVALIDATE_RANGE_BIT for mapping buffer objects,
which is translated to PIPE_MAP_DISCARD_RANGE.  However, unextended
OpenGL doesn't define mapping textures.  There are two main sources of
image maps: our internal MapTextureImage() hook, and gbm_bo_map().

I've audited our internal MapTextureImage() calls, and while some do
pass PIPE_MAP_DISCARD_RANGE, almost all of them wholly overwrite the
mapped region, and those that care about combining with existing image
contents all pass PIPE_MAP_READ.  So this should work there.

GBM defines three flags: GBM_BO_TRANSFER_READ, WRITE, and READ_WRITE.
There is no defined "invalidate range" bit.  In issue #6020, Matthias
Treydte notes that this extra readback can cause performance problems,
and with iris's current interpretation, there's no way to avoid it.
During that discussion, Michel and Matthias both argued that
GBM_BO_TRANSFER_WRITE should invalidate the destination contents and
avoid the readback, while GBM_BO_TRANSFER_READ_WRITE would preserve it.

This patch makes iris follow that model for image mappings, removing
readback on staging maps for both detiling and stall avoidance, unless
PIPE_MAP_READ is passed.  I believe we can change this with impunity.

For buffer objects, Ian Romanick and I both agree that partial writes
should be supported, and GL_MAP_INVALIDATE_RANGE_BIT exists precisely
to indicate that you should spend effort preserving existing contents.
So we continue doing readback for buffers unless PIPE_MAP_DISCARD_RANGE
is flagged, for now.  While I think this is work, it also seems to be
undertested in the CTS and Piglit.

Closes: https://gitlab.freedesktop.org/mesa/mesa/-/issues/6020
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/19209>