drm/mgag200: Move mode-setting code into separate helper function

The mode-setting code is now located in mgag200_set_mode_regs(), sans
a few flags that will be moved in a later patch for clarity.

v2:
	* replace uint8_t with u8

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/20200515083233.32036-5-tzimmermann@suse.de