drm/amd/display: Don't use non-registered VUPDATE on DCE 6

drm/amd/display: Don't use non-registered VUPDATE on DCE 6

The VUPDATE interrupt isn't registered on DCE 6, so don't try
to use that.

This fixes a page flip timeout after sleep/resume on 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>