drm/docs: Move "scaling mode" property.

The "scaling mode" property has been moved to the DRM->Generic.
It has also had a list of supported drivers added to it.

Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Robert Foss <robert.foss@collabora.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1462216381-10160-1-git-send-email-robert.foss@collabora.com