drm/simple-helpers: Add missing includes

Add missing includes to pull in definitions for drm_crtc,
drm_plane and drm_encoder.

Signed-off-by: Noralf Trønnes <noralf@tronnes.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/20170122181117.8210-4-noralf@tronnes.org