drm/mgag200: drop use of drmP.h

Drop use of the deprecated drmP.h header file.
Replace with necessary include files to fix build.

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/20190623103542.30697-5-sam@ravnborg.org