iris: Require a 4K alignment for extra clear color BOs.

iris: Require a 4K alignment for extra clear color BOs.

Apparently, we've been requiring a 4K alignment for internally allocated
clear color addresses to work around some unknown issues.  There's a
comment to that effect in iris_resource_create_with_modifiers().

When importing a dmabuf and tacking on an additional clear color BO, we
only required an alignment of 1.  This wasn't a problem for a long time
because all BO allocations were naturally aligned to the 4K page size.

However, once we enabled suballocation, we were able to allocate "BOs"
at 256B granularity, making this no longer 4K aligned.  Increase the
alignment requirement to 4K to match the behavior of our normal
allocations and also our previous behavior.

Fixes Piglit's ext_image_dma_buf_import-intel-modifiers.

Closes: https://gitlab.freedesktop.org/mesa/mesa/-/issues/5482
Fixes: ce2e2296ab6 ("iris: Suballocate BO using the Gallium pb_slab mechanism")
Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Reviewed-by: Nanley Chery <nanley.g.chery@intel.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/13326>