drm/mgag200: Separate DRM and PCI functionality from each other

Moving the DRM driver structures from the middle of the PCI code to
the top of the file makes it more readable. Also remove an obsolete
comment.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20200605135803.19811-9-tzimmermann@suse.de