drm/i915: Undo rmw damage to gen3 error interrupt handler

drm/i915: Undo rmw damage to gen3 error interrupt handler

The gen2/gen3 irq code is supposed to be identical apart
from the 32bit vs. 16bit access width. The recent change
to intel_de_rmw() ruined that symmetry. Restore it to avoid
needless mental gymnastics when comparing the two codepaths.

And while at it remove the extra eir!=0 check that somehow
ended up in the gen2 codepath only.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230125185234.21599-3-ville.syrjala@linux.intel.com
Reviewed-by: Vinod Govindapillai <vinod.govindapillai@intel.com>