MAINTAINERS: Update drm.git URL

Now that the main DRM tree has moved to Gitlab, adjust the MAINTAINERS
git trees to reflect the location change.

Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Link: https://lore.kernel.org/r/20240226152123.131406-1-mripard@kernel.org


Signed-off-by: Maxime Ripard <mripard@kernel.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>