We had defined MAX_IMAGES as 8, which we used to size the array for
image push constant data. The comment there stated that this was for
gen8, but anv_nir_apply_pipeline_layout runs for all gens and writes
that array, asserting that we don't exceed that number of images,
which imposes a limit of MAX_IMAGES on all gens.

Furthermore, despite this, we are exposing up to 64 images per shader
stage on all gens, gen8 included.

This patch lowers the number of images we expose in gen8 to 8 and
keeps 64 images for gen9+ while making sure that only pre-SKL gens
use push constant space to handle images.

v2:
 - <= instead of < in the assert (Eric, Lionel)
 - Change the way the assertion is written (Eric)

v3:
 - Revert the way the assertion is written to the form it had in v1,
   the version in v2 was not equivalent and was incorrect. (Lionel)

v4:
 - gen9+ doesn't need push constants for images at all (Jason)

Cc: mesa-stable@lists.freedesktop.org
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>
Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com> (v3)