drm/sun4i: Make pitch even for GEM dumb alloc as per hardware constraint

Our hardware requires the pitch to be an even number when using YUV
formats with the frontend. Implement a driver-specific callback for GEM
dumb allocation that sets the pitch accordingly.

Since only the bpp is passed (and not the format), we cannot really
distinguish if this alignment is really required. Since it doesn't hurt
to align the pitch anyway, always do it.

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/20181123092515.2511-30-paul.kocialkowski@bootlin.com