MAINTAINERS: drm: add maintainers for DRM buddy allocator

MAINTAINERS: drm: add maintainers for DRM buddy allocator

The DRM buddy allocator is a shared DRM memory management
component used by multiple DRM drivers.

Matthew Auld and Arun Pravin have been actively involved in
maintaining this code, including patch review and functional
changes.

Add a dedicated MAINTAINERS entry to reflect the current
maintainership.

v2: Include drivers/gpu/drm/tests/drm_buddy_test.c file (Matthew).

Signed-off-by: Arunpravin Paneer Selvam <Arunpravin.PaneerSelvam@amd.com>
Acked-by: Matthew Auld <matthew.auld@intel.com>
Acked-by: Christian König <christian.koenig@amd.com>
Link: https://patch.msgid.link/20260112114022.315139-1-Arunpravin.PaneerSelvam@amd.com