drm: Remove legacy version of get_scanout_position()

The legacy version of get_scanout_position() was only useful while
drivers still used drm_driver.get_scanout_position(). With no such
drivers left, the related typedef and code can be removed

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200123135943.24140-23-tzimmermann@suse.de