i965: Use predicate enable bit for conditional rendering w/o stalling

i965: Use predicate enable bit for conditional rendering w/o stalling

Previously whenever a primitive is drawn the driver would call
_mesa_check_conditional_render which blocks waiting for the result of
the query to determine whether to render. On Gen7+ there is a bit in
the 3DPRIMITIVE command which can be used to disable the primitive
based on the value of a state bit. This state bit can be set based on
whether two registers have different values using the MI_PREDICATE
command. We can load these two registers with the pixel count values
stored in the query begin and end to implement conditional rendering
without stalling.

Unfortunately these two source registers were not in the whitelist of
available registers in the kernel driver until v3.19. This patch uses
the command parser version from intel_screen to detect whether to
attempt to set the predicate data registers.

The predicate enable bit is currently only used for drawing 3D
primitives. For blits, clears, bitmaps, copypixels and drawpixels it
still causes a stall. For most of these it would probably just work to
call the new brw_check_conditional_render function instead of
_mesa_check_conditional_render because they already work in terms of
rendering primitives. However it's a bit trickier for blits because it
can use the BLT ring or the blorp codepath. I think these operations
are less useful for conditional rendering than rendering primitives so
it might be best to leave it for a later patch.

v2: Use the command parser version to detect whether we can write to
    the predicate data registers instead of trying to execute a
    register load command.
v3: Simple rebase
v4: Changes suggested by Kenneth Graunke: Split the
    load_64bit_register function out to a separate patch so it can be
    a shared public function. Avoid calling
    _mesa_check_conditional_render if we've already determined that
    there's no query object. Some styling fixes.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>