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

[Ilija Hadzic]

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.