drm/amd/pm: Add caching for SystemMetrics table

Implement caching separately for SystemMetrics table from PMFW. The same
table could be used for multiple interfaces. Hence, cache it internally
to avoid multiple queries to the firmware. For SystemMetrics table, 5ms
cache interval is sufficient.

Signed-off-by: Lijo Lazar <lijo.lazar@amd.com>
Reviewed-by: Asad Kamal <asad.kamal@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>