drm/xe/migrate: Future-proof compressed PAT check

drm/xe/migrate: Future-proof compressed PAT check

Although all current Xe2 platforms support FlatCCS, we probably
shouldn't assume that will be universally true forever.  In the past
we've had platforms like PVC that didn't support compression, and the
same could show up again at some point in the future.  Future-proof the
migration code by adding an explicit check for FlatCCS support to the
condition that decides whether to use a compressed PAT index for
migration.

While we're at it, we can drop the IS_DGFX check since it's redundant
with the src_is_vram check (only dGPUs have VRAM).

Cc: Akshata Jahagirdar <akshata.jahagirdar@intel.com>
Cc: Lucas De Marchi <lucas.demarchi@intel.com>
Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Reviewed-by: Akshata Jahagirdar <akshata.jahagirdar@intel.com>
Reviewed-by: Lucas De Marchi <lucas.demarchi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240726171757.2728819-2-matthew.d.roper@intel.com