Add a note to MAINTAINERS for lkpi for graphics

Add a note to MAINTAINERS requesting pre-commit review from the graphics
team, using phabricator, for changes to the lkpi subsystem. This is done in
order to give us a chance to test the graphics drivers (drm drivers) for
regressions, and to try to avoid breakage, errors and issues with the
graphics drivers.
The review is done via the x11 group on phabricator.

Please note that hselasky also want to review changes.

Discussed with: hselasky, imp
Approved by: imp