drm/i915: Mask page table errors on gen2/3 with FBC

drm/i915: Mask page table errors on gen2/3 with FBC

FBC on gen2/3 seems to trigger page table errors. No visual
artifacts are visible, and essentially the same FBC
code works on gen4 so these seem entirely spurious. There
are also hints in gen3 bspec indicating that certain bits
in PGTBL_ER are just not wired up correctly in the
hardware.

Ideally we'd want to mask out only the bogus bits, but
sadly there is no mask for PGTBL_ER, and instead we are
forced to mask out all page table errors via EMR :(

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230125185234.21599-6-ville.syrjala@linux.intel.com
Reviewed-by: Vinod Govindapillai <vinod.govindapillai@intel.com>