drm/fb-helper: Call dirty helper after writing to fbdev

If fbdev uses a shadow framebuffer, call the damage handler. Otherwise
the update might not make it to the screen.

v2:
	* mark virtual screen as dirty (Ville)

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Fixes: 222ec45f ("drm/fb_helper: Support framebuffers in I/O memory")
Acked-by: Maxime Ripard <mripard@kernel.org>
Acked-by: Sam Ravnborg <sam@ravnborg.org>
Cc: Thomas Zimmermann <tzimmermann@suse.de>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Sam Ravnborg <sam@ravnborg.org>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Maxime Ripard <mripard@kernel.org>
Cc: David Airlie <airlied@linux.ie>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: Gerd Hoffmann <kraxel@redhat.com>
Cc: dri-devel@lists.freedesktop.org
Cc: virtualization@lists.linux-foundation.org
Link: https://patchwork.freedesktop.org/patch/msgid/20201120102545.4047-2-tzimmermann@suse.de