drm: WARN when trying add enum values to non-enum/bitmask properties

Trying to add enum values to non-enum/bitmask properties is a
programmer mistake. WARN to make sure the developers notice
their mistake.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180306164849.2862-2-ville.syrjala@linux.intel.com


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