drm/omap: Potential NULL deref in omap_crtc_duplicate_state()

If the kmalloc() fails then we dereference "state" when we set
"state->zpos".

Fixes: 3dfeb631

 ("drm/omap: Rework the rotation-on-crtc hack")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>