drm: add "auto" dithering method

There's no convenient/reliable way for drivers to both obey the dithering
mode property, and to be able to attempt to provide a good default in all
cases.

This commit adds an "auto" method to the property which drivers can default
to if they wish, whilst still allowing the user to override the choice as
they do now.

Signed-off-by: Ben Skeggs <bskeggs@redhat.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>