drm/mgag200: Use simple-display data structures

The MGA CRTC data structure struct mga_crtc contains unused additional
fields; so it can removed. The standard DRM CRTC and encoder structures
are embedded now in struct drm_simple_display_pipe. Done in preparation
of converting mgag200 to simple KMS helpers.

v3:
	* remove now-unused define MGAG200FB_CONN_LIMIT
	* remove unused define MATROX_DPMS_CLEARED

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/20200515083233.32036-14-tzimmermann@suse.de