drm: WARN() when drm_connector_attach_property fails

Also return void instead of int. We have more than 100 callers and
no one checks for the return value.

If this function fails the property won't be exposed by the get/set
ioctls, but we should probably survive. If this starts happening,
the solution will be to increase DRM_CONNECTOR_MAX_PROPERTY and
recompile the Kernel.

Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Reviewed-by: Rob Clark <rob.clark@linaro.org>
Tested-by: Rob Clark <rob.clark@linaro.org>
Signed-off-by: Dave Airlie <airlied@redhat.com>