MAINTAINERS: Add Tvrtko as drm/i915 co-maintainer

Add Tvrtko Ursulin as a co-maintainer for drm/i915 driver.
Tvrtko will bring added bandwidth and focus to the GT/GEM domain
(drm-intel-gt-next).

This will help with the increased driver maintenance efforts, allows
alternating the drm-intel-gt-next pull requests and also should increase
the coverage for the maintenance in general.

Signed-off-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: David Airlie <airlied@linux.ie>
Cc: Daniel Vetter <daniel@ffwll.ch>
Acked-by: Jani Nikula <jani.nikula@linux.intel.com>
Acked-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Acked-by: Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>
Cc: Sean Paul <sean@poorly.run>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Maxime Ripard <mripard@kernel.org>
Cc: dri-devel@lists.freedesktop.org
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Acked-by: Dave Airlie <airlied@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20211025134907.20078-1-joonas.lahtinen@linux.intel.com