drm/fb-helper: Copy dma-buf map before flushing shadow fb

Copy the vmap()'ed instance of struct dma_buf_map before modifying it,
in case the implementation of vunmap() depends on the exact address.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Maxime Ripard <mripard@kernel.org>
Acked-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20201120102545.4047-10-tzimmermann@suse.de