amdgpu/pm: Make sysfs pm attributes as read-only for VFs

amdgpu/pm: Make sysfs pm attributes as read-only for VFs

== Description ==
Setting values of pm attributes through sysfs
should not be allowed in SRIOV mode.
These calls will not be processed by FW anyway,
but error handling on sysfs level should be improved.

== Changes ==
This patch prohibits performing of all set commands
in SRIOV mode on sysfs level.
It offers better error handling as calls that are
not allowed will not be propagated further.

== Test ==
Writing to any sysfs file in passthrough mode will succeed.
Writing to any sysfs file in ONEVF mode will yield error:
"calling process does not have sufficient permission to execute a command".

Signed-off-by: Marina Nikolic <Marina.Nikolic@amd.com>
Acked-by: Evan Quan <evan.quan@amd.com>
Reviewed-by: Lijo Lazar <lijo.lazar@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>