drm: fix one flawed mutex grab and remove some spurious mutex grabs

The following three patches remove unecessary locks around ioctls in drm module.

First two:

[PATCH 1/3] drm: no need to hold global mutex for static data [PATCH 2/3] drm: make DRM_UNLOCKED ioctls with their own mutex

are rather trivial and straight forward and probably do not need much explanation.

The third one:

[PATCH 3/3] drm: do not sleep on vblank while holding a mutex

is more serious and clears a clog that can occur if multiple processes call drm_wait_vblank as explained in patch commit message.