drm/mga: make header file self contained

This makes migration away from drmP.h simple
as we do not need to duplicate dependencies required by mga_drv.h

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Acked-by: Thomas Zimmermann <tzimmermann@suse.de>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: David Airlie <airlied@linux.ie>
Link: https://patchwork.freedesktop.org/patch/msgid/20190623103542.30697-3-sam@ravnborg.org