drm/amd/display: Disable VRR on DCE 6

drm/amd/display: Disable VRR on DCE 6

DCE 6 was not advertised as being able to support VRR,
so let's mark it as unsupported for now.

The VRR implementation in amdgpu_dm depends on the VUPDATE
interrupt which is not registered for DCE 6.

Signed-off-by: Timur Kristóf <timur.kristof@gmail.com>
Reviewed-by: Rodrigo Siqueira <siqueira@igalia.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Alex Hung <alex.hung@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>