drm/amd/display: add mod_freesync_user_enable to dm_connector_state

We don't currently expose variable refresh rate, but add the state to the
connector state to make it easier to maintain the support for it from a
hw support perspective while we figure out the uapi for drm.

Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>