drm/amd/display: Fix p-state allow debug index on dcn31

drm/amd/display: Fix p-state allow debug index on dcn31

[Why]
It changed since dcn30 but the hubbub31 constructor hasn't been
modified to reflect this.

[How]
Update the value in the constructor to 0x6 so we're checking the right
bits for p-state allow.

It worked before by accident, but can falsely assert 0 depending on HW
state transitions. The most frequent of which appears to be when
all pipes turn off during IGT tests.

Cc: Harry Wentland <harry.wentland@amd.com>
Fixes: e7031d8258f1b4 ("drm/amd/display: Add pstate verification and recovery for DCN31")
Signed-off-by: Nicholas Kazlauskas <nicholas.kazlauskas@amd.com>
Reviewed-by: Eric Yang <Eric.Yang2@amd.com>
Acked-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>