drm/i915: Extract {i9xx,i965)_error_mask()

Pull the EMR calculation into small helpers.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230125185234.21599-5-ville.syrjala@linux.intel.com


Reviewed-by: Vinod Govindapillai <vinod.govindapillai@intel.com>