MAINTAINERS: dri-devel is for all of drivers/gpu

Somehow we only have a list of subdirectories, which apparently made
it harder for folks to find the gpu maintainers. Fix that.

References: https://lore.kernel.org/dri-devel/YXrAAZlxxStNFG%2FK@phenom.ffwll.local/
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Cc: David Airlie <airlied@linux.ie>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: Steven Rostedt <rostedt@goodmis.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20211028170857.4029606-1-daniel.vetter@ffwll.ch