iris: Clean up iris_sample_with_depth_aux()

iris: Clean up iris_sample_with_depth_aux()

The majority of the logic here was for the Gfx8-9 sample-from-hiz
hardware feature, which only applies to AUX_USAGE_HIZ, and neither
of the combined HiZ+CCS modes.  So, reorganize the function to put
the logic for each case in the case itself.

Reviewed-by: Nanley Chery <nanley.g.chery@intel.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4674>