MAINTAINERS: Updates to Intel DRM

Introduce the Maintainers of the new drm/xe driver for upcoming
Intel GPUs.

Since it has a shared display with drm/i915, let's also create a
dedicated block to group display related files. But without any
substantial change to the i915 side. The display patches will
continue to flow through i915 from drm-intel-next branches for now.

Acked-by: Jani Nikula <jani.nikula@intel.com>
Acked-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Acked-by: Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>
Acked-by: Lucas De Marchi <lucas.demarchi@intel.com>
Acked-by: Oded Gabbay <ogabbay@kernel.org>
Acked-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Acked-by: Dave Airlie <airlied@redhat.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>