drm/selftests: Add drm helper selftest

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180503112217.37292-6-maarten.lankhorst@linux.intel.com


Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>