drm/amd/display: Use DRAM speed from validation for dummy p-state

[Description]
When choosing which dummy p-state latency to use, we
need to use the DRAM speed from validation. The DRAMSpeed
DML variable can change because we use different input
params to DML when populating watermarks set B.

Cc: stable@vger.kernel.org # 6.1+
Reviewed-by: Samson Tam <samson.tam@amd.com>
Acked-by: Hamza Mahfooz <hamza.mahfooz@amd.com>
Signed-off-by: Alvin Lee <alvin.lee2@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>