drm/amd/pm/powerplay/hwmgr/vega20_thermal: Fix some outdated function documentation

Fixes the following W=1 kernel build warning(s):

 drivers/gpu/drm/amd/amdgpu/../pm/powerplay/hwmgr/vega20_thermal.c:217: warning: Function parameter or member 'hwmgr' not described in 'vega20_thermal_get_temperature'
 drivers/gpu/drm/amd/amdgpu/../pm/powerplay/hwmgr/vega20_thermal.c:242: warning: Function parameter or member 'hwmgr' not described in 'vega20_thermal_set_temperature_range'
 drivers/gpu/drm/amd/amdgpu/../pm/powerplay/hwmgr/vega20_thermal.c:242: warning: Function parameter or member 'range' not described in 'vega20_thermal_set_temperature_range'
 drivers/gpu/drm/amd/amdgpu/../pm/powerplay/hwmgr/vega20_thermal.c:278: warning: Function parameter or member 'hwmgr' not described in 'vega20_thermal_enable_alert'
 drivers/gpu/drm/amd/amdgpu/../pm/powerplay/hwmgr/vega20_thermal.c:296: warning: Function parameter or member 'hwmgr' not described in 'vega20_thermal_disable_alert'
 drivers/gpu/drm/amd/amdgpu/../pm/powerplay/hwmgr/vega20_thermal.c:310: warning: Function parameter or member 'hwmgr' not described in 'vega20_thermal_stop_thermal_controller'
 drivers/gpu/drm/amd/amdgpu/../pm/powerplay/hwmgr/vega20_thermal.c:326: warning: Function parameter or member 'hwmgr' not described in 'vega20_thermal_setup_fan_table'

Cc: Evan Quan <evan.quan@amd.com>
Cc: Alex Deucher <alexander.deucher@amd.com>
Cc: "Christian König" <christian.koenig@amd.com>
Cc: David Airlie <airlied@linux.ie>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: amd-gfx@lists.freedesktop.org
Cc: dri-devel@lists.freedesktop.org
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>