drm: Simplify return value handling in drm_crtc.c

While looking through drm_crtc.c to double-check make locking changes
I've noticed that there's a few other places that would now benefit
from simplified return value handling.

So let's flatten the control flow and replace and always 0 ret with 0
where possible.

Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Reviewed-by: Sean Paul <seanpaul@chromium.org>
Signed-off-by: Dave Airlie <airlied@redhat.com>