drm/imx: Drop drm_vblank_cleanup

It's only done in the driver load error path, where vblanks don't need
to be quiescent anyway. And that's all drm_vblank_cleanup does, since
the core will release the vblank allocations on its own already. So
drop it.

Cc: Philipp Zabel <p.zabel@pengutronix.de>
Acked-by: Philipp Zabel <p.zabel@pengutronix.de>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170524145212.27837-24-daniel.vetter@ffwll.ch