drm/mgag200: Convert struct drm_device to struct mga_device with helper

Mgag200 uses dev_private to look up struct mga_device for instances
of struct drm_device. Use of dev_private is deprecated, so hide it in
the helper function to_mga_device().

v2:
	* make to_mga_device() a function (Sam)

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20200507090315.21274-2-tzimmermann@suse.de