drm/mgag200: Move register initialization into separate function

Registers are initialized with constants. This is now done in
mgag200_init_regs(), mgag200_set_dac_regs() and mgag200_set_pci_regs().
Later patches should move these calls from mode setting to device
initialization.

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/20200515083233.32036-12-tzimmermann@suse.de