drm/mgag200: Split MISC register update into PLL selection, SYNC and I/O

Set different fields in MISC in their rsp location in the code. This
patch also fixes a bug in the original code where the mode's SYNC flags
were never written into the MISC register.

v2:
	* use u8 instead of uint8_t
	* define MGAREG_MISC_CLK_SEL_MASK

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/20200515083233.32036-6-tzimmermann@suse.de