drm/amd/pm: add set_performance_level function for yellow carp

This patch enables editing sysfs file power_dpm_force_performance_level
for yellow carp. User could thus adjust the dpm forced level.

v2: enable fine grain control of GFXCLK only when in manual performance
level mode. In other mode, the min/max range of GFXCLK will be reset to
corresponding values.

Ex: To enable min 300MHz / max 600MHz gfx clocks

1) echo manual > /sys/bus/pci/devices/.../power_dpm_force_performance_level
2) echo s 0 300 > /sys/bus/pci/devices/.../pp_od_clk_voltage
3) echo s 1 600 > /sys/bus/pci/devices/.../pp_od_clk_voltage
4) echo c > /sys/bus/pci/devices/.../pp_od_clk_voltage

Signed-off-by: Xiaomeng Hou <Xiaomeng.Hou@amd.com>
Acked-by: Huang Rui <ray.huang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>