drm/amd/display: Update TTU properly

[Why]
We need to update TTU properly if DRAMClockChangeWatermark changes. If
TTU < DRAMClockChangeWatermark, we pstate won't be allowed and we will
hang in some PSR cases.

[How]
Update TTU if DramClockChangeWatermark value increases (only if TTU was
dependent on the watermark value on the DRAMClockChangeWatermark value
in the first place).

Signed-off-by: Alvin Lee <alvin.lee2@amd.com>
Reviewed-by: Jun Lei <Jun.Lei@amd.com>
Acked-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Acked-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>