drm/mgag200: Use offset-adjusted shadow-plane mappings

For framebuffers with non-zero offset fields, shadow-plane helpers
provide a pointer to the first byte of the contained data. Use it in
mgag200.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20210803125928.27780-6-tzimmermann@suse.de