drm: Remove drm_vblank_cleanup from drm_vblank_init error path.

drm_vblank_cleanup() would operate on non-existent dev->vblank
data structure, as failure to allocate that data structure is
what triggers the error path in the first place.

Signed-off-by: Mario Kleiner <mario.kleiner.de@gmail.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>