drm: make the connector properties code use the object properties code

In the future, we may want to kill the internal functions:
- drm_connector_attach_property
- drm_connector_property_set_value
- drm_connector_property_get_value

It seems the IOCTL drm_mode_connector_property_set_ioctl will have to live, but
we may change libdrm to not use it anymore, if we want.

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