drm: do not sleep on vblank while holding a mutex

drm_wait_vblank must be DRM_UNLOCKED because otherwise it
will grab the drm_global_mutex and then go to sleep until the vblank
event it is waiting for. That can wreck havoc in the windowing system
because if one process issues this ioctl, it will block all other
processes for the duration of all vblanks between the current and the
one it is waiting for. In some cases it can block the entire windowing
system.

v2: incorporate comments received from Daniel Vetter and
    Michel Daenzer.

v3/v4: after a lengty discussion with Daniel Vetter, it was concluded
       that the only thing not yet protected with locks and atomic
       ops is the write to dev->last_vblank_wait. It's only used in a
       debug file in proc, and the current code already employs no
       correct locking: the proc file only takes dev->struct_mutex,
       whereas drm_wait_vblank implicitly took the drm_global_mutex.
       Given all this, it's not worth bothering to try to fix
       the locks at this time.

Signed-off-by: Ilija Hadzic <ihadzic@research.bell-labs.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Dave Airlie <airlied@redhat.com>