drm/xe: Don't hardcode GuC's MOCS index in register header

Although PVC is currently the only platform that needs us to program a
GuC register with the index of an uncached MOCS entry, it's likely other
platforms will need this in the future.  Rather than hardcoding PVC's
index into the register header, we should just pull the appropriate
index from gt->mocs.uc_index to future-proof the code.

Reviewed-by: Matthew Brost <matthew.brost@intel.com>
Link: https://lore.kernel.org/r/20230602235210.1314028-3-matthew.d.roper@intel.com


Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>