drm/mgag200: Set pitch in a separate helper function

The framebuffer's pitch is now set in mgag200_set_offset().

v2:
	* move offset and bpp-shift calculation into helper functions
	* use u8 instead of uint8_t
	* add MGAREG_CRTCEXT0_OFFSET_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/20200515083233.32036-8-tzimmermann@suse.de