drm/sun4i: Make COEF_RDY conditional

The COEF_RDY bit isn't found in all the SoCs featuring some variant of the
frontend.

Add it to our quirks structure.

Reviewed-by: Paul Kocialkowski <paul.kocialkowski@bootlin.com>
Signed-off-by: Maxime Ripard <maxime.ripard@bootlin.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190118145133.21281-21-paul.kocialkowski@bootlin.com