MAINTAINERS: add Andrey as the DRM GPU scheduler maintainer

Now that the scheduler is being used by more and more
drivers, we need someone to maintain it.  Andrey has
stepped up to maintain the scheduler.

Cc: Andrey Grodzovsky <andrey.grodzovsky@amd.com>
Cc: airlied@gmail.com
Cc: daniel.vetter@ffwll.ch
Reviewed-by: Christian König <christian.koenig@amd.com>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210917161540.822282-2-alexander.deucher@amd.com