amdgpu/pm: Disable managing hwmon sysfs attributes for ONEVF mode

This patch prohibits performing of set commands on all hwmon attributes
through sysfs in ONEVF mode.

Signed-off-by: Danijel Slivka <danijel.slivka@amd.com>
Acked-by: Harish Kasiviswanathan <Harish.Kasiviswanathan@amd.com>
Reviewed-by: Harish Kasiviswanathan <Harish.Kasiviswanathan@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>