drm/vc4: Convert to struct drm_crtc_helper_funcs.get_scanout_position()

The callback struct drm_driver.get_scanout_position() is deprecated in
favor of struct drm_crtc_helper_funcs.get_scanout_position(). Convert vc4
over.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Eric Anholt <eric@anholt.net>
Link: https://patchwork.freedesktop.org/patch/msgid/20200123135943.24140-18-tzimmermann@suse.de