drm/amdgpu/display: use blanked rather than plane state for sync groups

We may end up with no planes set yet, depending on the ordering, but we
should have the proper blanking state which is either handled by either
DPG or TG depending on the hardware generation.  Check both to determine
the proper blanked state.

Bug: https://gitlab.freedesktop.org/drm/amd/issues/781


Fixes: 5fc0cbfa ("drm/amd/display: determine if a pipe is synced by plane state")
Cc: nicholas.kazlauskas@amd.com
Reviewed-by: Nicholas Kazlauskas <nicholas.kazlauskas@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org