drm/mgag200: Prefix symbol names in mgag200_drv.c with mgag200_

The naming of symbols in mgag200_drv.c is inconsistent. Fix that by
prefixing all names with mgag200_.

v2:
	* clarify commit message

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20200605135803.19811-10-tzimmermann@suse.de