drm/i915: Nuke atomic set/get prop plane stubs

They have been unused since rotation was added to drm core in 2015,
time to get rid of them.

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190611132820.31981-1-maarten.lankhorst@linux.intel.com


Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>