modesetting: Create CONNECTOR_ID properties for outputs [v2]

This lets a DRM client map between X outputs and kernel connectors.

v2:
	Change CONNECTOR_ID to enum -- Adam Jackson <ajax@nwnk.net>

Signed-off-by: Keith Packard <keithp@keithp.com>
(Ported from xserver commit 023d4aba8d45e9e3630b944ecfb650c081799b96)
Signed-off-by: Michel Dänzer <michel.daenzer@amd.com>
(Ported from amdgpu commit ab7e39c5a03e24c3ce3ee2f22ada7572bc2d9aa7)
Acked-by: Alex Deucher <alexander.deucher@amd.com>