drm/amd/powerplay: add control method to bypass metrics cache on Vega20

As for the gpu metric export, metrics cache makes no sense. It's up to
user to decide how often the metrics should be retrieved.

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>