drm/amd/pm: Add sysfs attribute to get pm metrics

Add sysfs attribute to read power management metrics. A snapshot is
captured to the buffer when the attribute is read.

Signed-off-by: Lijo Lazar <lijo.lazar@amd.com>
Reviewed-by: Hawking Zhang <Hawking.Zhang@amd.com>
Reviewed-by: Yang Wang <kevinyang.wang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>