drm/amd/powerplay: add new UMD pstate data structure

This is used to cache the clock frequencies for all UMD pstates.
So that we do not need to calculate from scratch on every UMD
pstate switch.

Signed-off-by: Evan Quan <evan.quan@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>