intel/isl: Switch to R8_UNORM format for compatiblity

intel/isl: Switch to R8_UNORM format for compatiblity

Gen12 added CCS_E support for A8_UNORM. Intercept A8_UNORM format and
switch to R8_UNORM, as both share the same aux map format encoding so
they are compatible.

Fixes Piglit's ext_framebuffer_multisample-formats all_samples, which
was hitting an assert about A8_UNORM and R8_UINT not being CCS_E
compatible formats.

v2: Add gen check (Kenneth Graunke)

v3: Intercept A8_UNORM and set format to R8_UNORM (Jason Ekstrand)

v4:
- Remove gen check and move block little bit down (Jason Ekstrand)

Signed-off-by: Sagar Ghuge <sagar.ghuge@intel.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>
Tested-by: Marge Bot <https://gitlab.freedesktop.org/mesa/mesa/merge_requests/3719>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/merge_requests/3719>