drm/mgag200: Move vmap out of commit tail

Vmap operations may acquire the dmabuf reservation lock, which is not
allowed within atomic commit-tail functions. Therefore move vmap and
vunmap from the damage handler into prepare_fb and cleanup_fb callbacks.

The mapping is provided as GEM shadow-buffered plane. The functions in
the commit tail use the pre-established mapping for damage handling.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Tested-by: Gerd Hoffmann <kraxel@redhat.com>
Acked-by: Gerd Hoffmann <kraxel@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210208115538.6430-5-tzimmermann@suse.de