drm/omap: cleanup omap_plane_atomic_check()

Clean up omap_plane_atomic_check() with:

- Check state->fb first. If no fb, return 0.
- use drm_atomic_get_existing_crtc_state() instead of
  drm_atomic_get_crtc_state()

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>