drm/amd/display: Fix type of pp_smu_wm_set_range struct

[why]
Value read from SMU is 16 bits, not 32.

[How]
Fix type, and add wm_type enum in preparation for future interfaces.

Signed-off-by: Eric Yang <Eric.Yang2@amd.com>
Reviewed-by: Tony Cheng <Tony.Cheng@amd.com>
Acked-by: Leo Li <sunpeng.li@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>