drm/radeon: Use rdev->gem.mutex to protect hyperz/cmask owners

This removes the last depency of radeon for dev->struct_mutex!

Also the locking scheme for hyperz/cmask owners seems a bit unsound,
there's no protection in the preclose handler (and that never did hold
dev->struct_mutex while being called). So grab the same lock there,
too.

There's also all the checks in the cs checker, but since the overall
design seems to never stall for the previous owner I figured it's ok
if I leave this racy. It was racy even before I touched it after all
too.

Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>