drm/sun4i: tcon: Support an active-low DE signal with RGB interface

Some panels need an active-low data enable (DE) signal for the RGB
interface. This requires flipping a bit in the TCON0 polarity register
when setting up the mode for the RGB interface.

Match the associated bus flag and use it to set the polarity inversion
bit for the DE signal when required.

Signed-off-by: Paul Kocialkowski <contact@paulk.fr>
Signed-off-by: Maxime Ripard <maxime.ripard@bootlin.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181107181843.27628-4-contact@paulk.fr