drm/amd/display: Disable uclk pstate for subvp pipes

[Description]
When subvp is in use, main pipes should block unintended natural uclk pstate
changes to prevent disruption to the state machine.

Reviewed-by: Alvin Lee <Alvin.Lee2@amd.com>
Acked-by: Stylon Wang <stylon.wang@amd.com>
Signed-off-by: Dillon Varone <Dillon.Varone@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>