drm/mgag200: Remove several references to struct mga_device.dev

Done in preparation of embedding the DRM device in struct mga_device.
This patch makes the patch for embedding more readable.

v2:
	* improved commit message (Daniel)

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/20200507090315.21274-3-tzimmermann@suse.de