amdgpu/pm: modify and add smu_get_power_limit to Powerplay API

 modify args of smu_get_power_limit to match Powerplay API .get_power_limit
 add smu_get_power_limit to Powerplay API swsmu_pm_funcs
 remove special handling of smu in amdgpu_hwmon_show_power_cap*

* Test
 AMDGPU_PCI_ADDR=`lspci -nn | grep "VGA\|Display" | cut -d " " -f 1`
 AMDGPU_HWMON=`ls -la /sys/class/hwmon | grep $AMDGPU_PCI_ADDR | cut -d " " -f 11`
 HWMON_DIR=/sys/class/hwmon/${AMDGPU_HWMON}

 lspci -nn | grep "VGA\|Display" ; \
 echo "=== power1 cap ===" ; cat $HWMON_DIR/power1_cap ;           \
 echo "=== power1 cap max ===" ; cat $HWMON_DIR/power1_cap_max ;   \
 echo "=== power1 cap def ===" ; cat $HWMON_DIR/power1_cap_default

Signed-off-by: Darren Powell <darren.powell@amd.com>
Reviewed-by: Lijo Lazar <lijo.lazar@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>