drm/amd/display: Add dummy p-state latency bounding box override

[Why]

For debugging, it can be useful to be able to modify the dummy
p-state latency, this will make it easier to do so.

Signed-off-by: Joshua Aberback <joshua.aberback@amd.com>
Reviewed-by: Wesley Chalmers <Wesley.Chalmers@amd.com>
Acked-by: Aurabindo Pillai <aurabindo.pillai@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>