drm/sun4i: move rgb mode_valid from connector to encoder

mode_valid function must be connected to encoder.
Otherwise it could get not be called by drm in the case there's a
bridge connected to encoder instead of a panel.

Move mode_valid function pointer to encoder helper functions,
changing its prototype according to encoder helper function pointer.

Signed-off-by: Giulio Benetti <giulio.benetti@micronovasrl.com>
Signed-off-by: Maxime Ripard <maxime.ripard@bootlin.com>
Link: https://patchwork.freedesktop.org/patch/msgid/1520941017-81177-1-git-send-email-giulio.benetti@micronovasrl.com