drm/nouveau: remove master

drm/nouveau: remove master

The only remaining nouveau_drm.master struct member that's being used is
the mutex that protects its object tree.  Move that into nouveau_drm and
remove nouveau_drm.master entirely.

A pending series to remove the "ioctl" layer between DRM and NVKM also
removes the need for object handle lookups, and hence this mutex, but
it's still required for the moment.

Signed-off-by: Ben Skeggs <bskeggs@nvidia.com>
Signed-off-by: Danilo Krummrich <dakr@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20240726043828.58966-35-bskeggs@nvidia.com