drm/fb-helper: Separate shadow-buffer flushing and calling dirty callback

Flushing the shadow framebuffer and invoking the dirty callback are two
separate operations, so do them separately. The flush operation is paired
with calls to vmap and vunmap. They are not needed for the dirty callback,
which performs its own invocations if necessary.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Sam Ravnborg <sam@ravnborg.org>
Acked-by: Maxime Ripard <mripard@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20201120102545.4047-7-tzimmermann@suse.de