drm/mgag200: Inline mga_crtc_{prepare, commit}() into enable function

There's only trivial code left in mga_crtc_{prepare,commit}(). Merge the
functions into the simple pipe's enable function.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Lyude Paul <lyude@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200707082411.6583-8-tzimmermann@suse.de