drm/mgag200: Introduce custom CRTC state

Inherit from struct drm_crtc_state by embeding it and providing the
rsp callbacks for simple-kms helpers. No functional changes.

The new state struct mgag200_crtc_state will hold PLL values for modeset
operations.

v2:
	* move the simple-kms changes into a separate patch (Sam)

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20210714142240.21979-13-tzimmermann@suse.de