drm: create struct drm_object_properties and use it

For now, only connectors have it. In the future, all objects that need
properties should use it. Since the structure is referenced inside
struct drm_mode_object, we will be able to deal with object properties
without knowing the real type of the object.

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>