drm: Add vrr_capable property to the drm connector

Modern display hardware is capable of supporting variable refresh rates.
This patch introduces the "vrr_capable" property on the connector to
allow userspace to query support for variable refresh rates.

Atomic drivers should attach this property to connectors that are
capable of driving variable refresh rates using
drm_connector_attach_vrr_capable_property().

The value should be updated based on driver and hardware capability
by using drm_connector_set_vrr_capable_property().

Signed-off-by: Nicholas Kazlauskas <nicholas.kazlauskas@amd.com>
Reviewed-by: Manasi Navare <manasi.d.navare@intel.com>
Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>