drm/amd: Add a new hwmon attribute for instantaneous power

Some GPUs provide support for current power, some average power,
and some both.  To be able to support all these combinations,
introduce a new attribute.

Signed-off-by: Mario Limonciello <mario.limonciello@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>