drm/amd/display: Clear cached watermark after resume

[WHY]
Driver could skip program watermarks when resume from S0i3/S4.

[HOW]
Clear the cached one first to make sure new value gets applied.

Reviewed-by: Alvin Lee <alvin.lee2@amd.com>
Reviewed-by: Roman Li <roman.li@amd.com>
Signed-off-by: Charlene Liu <Charlene.Liu@amd.com>
Signed-off-by: Alex Hung <alex.hung@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>