drm/amdgpu: Add sysfs interfaces for NPS mode

Add a sysfs interface to see available NPS modes to switch to -

	cat /sys/bus/pci/devices/../available_memory_paritition

Make the current_memory_partition sysfs node read/write for requesting a
new NPS mode. The request is only cached and at a later point a driver
unload/reload is required to switch to the new NPS mode.

Ex:
	echo NPS1 > /sys/bus/pci/devices/../current_memory_paritition
	echo NPS4 > /sys/bus/pci/devices/../current_memory_paritition

The above interfaces will be available only if the SOC supports more than
one NPS mode.

Also modify the current memory partition sysfs logic to be more
generic.

Signed-off-by: Lijo Lazar <lijo.lazar@amd.com>
Reviewed-by: Rajneesh Bhardwaj <rajneesh.bhardwaj@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>