linux-imx

drm: Fix drm_crtc_vblank_get() documentation

drm_crtc_vblank_get() is the new drm_vblank_get(), not the new
drm_vblank_off().

Signed-off-by: Damien Lespiau <damien.lespiau@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>