drm/mgag200: Set primary plane's format in separate helper function

The primary plane's format registers are now updated in a
mgag200_set_format_regs().

v2:
	* get bpp shift from helper function
	* 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/20200515083233.32036-9-tzimmermann@suse.de