drm/amd/display: temp w/a for DP Link Layer compliance

[Why&How]
Disabling P-State support on full updates for DCN401 results in
introducing additional communication with SMU. A UCLK hard min message
to SMU takes 4 seconds to go through, which was due to DCN not allowing
pstate switch, which was caused by incorrect value for TTU watermark
before blanking the HUBP prior to DPG on for servicing the test request.

Fix the issue temporarily by disallowing pstate changes for compliance
test while test request handler is reworked for a proper fix.

Fixes: 67ea53a4 ("drm/amd/display: Disable DCN401 UCLK P-State support on full updates")
Cc: Mario Limonciello <mario.limonciello@amd.com>
Cc: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Dillon Varone <dillon.varone@amd.com>
Signed-off-by: Aurabindo Pillai <aurabindo.pillai@amd.com>
Signed-off-by: Wayne Lin <wayne.lin@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
(cherry picked from commit 8a79f7cd)
Cc: stable@vger.kernel.org