drm/komeda: Potential error pointer dereference

We need to check whether drm_atomic_get_crtc_state() returns an error
pointer before dereferencing "crtc_st".

Fixes: 9e560309

 ("drm/komeda: Add komeda_plane/plane_helper_funcs")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: "james qian wang (Arm Technology China)" <james.qian.wang@arm.com>
Signed-off-by: Liviu Dudau <liviu.dudau@arm.com>