drm/mgag200: Move TAGFIFO reset into separate function

The TAGFIFO state is now reset in mgag200_g200er_reset_tagfifo().

v2:
	* define MGAREG_SEQ1_SCROFF

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Tested-by: John Donnelly <John.p.donnelly@oracle.com>
Acked-by: Sam Ravnborg <sam@ravnborg.org>
Acked-by: Emil Velikov <emil.velikov@collabora.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200515083233.32036-10-tzimmermann@suse.de