drm/xe: Drop GFX_FLSH_CNTL_GEN6 write during GGTT invalidation

The write of GFX_FLSH_CNTL_GEN6 was inherited from the i915 codebase
where it was used to force a flush of the write-combine buffer in cases
where the GSM/GGTT were mapped as WC.  Since Xe never uses WC mappings
of the GGTT, this register write is unnecessary.  Furthermore, this
register was removed on Xe_HP-based platforms, so this write winds up
clobbering an unrelated register.

v2:
 - Also drop GFX_FLSH_CNTL_GEN6 from the register file now that it's no
   longer used.  (Lucas)

Reviewed-by: Lucas De Marchi <lucas.demarchi@intel.com>
Link: https://lore.kernel.org/r/20230418230247.3802438-1-matthew.d.roper@intel.com


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