drm: Reject replacing property enum values

drm: Reject replacing property enum values

If the property already has the enum value WARN and bail.
Replacing enum values doesn't make sense to me.

Throw out the pointless list_empty() while at it.

Cc: Daniel Vetter <daniel@ffwll.ch>
Suggested-by: Daniel Vetter <daniel@ffwll.ch>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180306164849.2862-1-ville.syrjala@linux.intel.com
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>