drm/sun4i: backend: Avoid counting YUV planes that use the frontend

Our hardware has a limited number of YUV planes (usually 1) that can be
supported using the backend only. However, YUV planes can also be
supported by the frontend and must then not be counted when checking for
that limitation.

Only count the YUV plane when the frontend is not used.

Signed-off-by: Paul Kocialkowski <paul.kocialkowski@bootlin.com>
Signed-off-by: Maxime Ripard <maxime.ripard@bootlin.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181123092515.2511-15-paul.kocialkowski@bootlin.com