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

 drivers/gpu/drm/amd/amdgpu/../pm/powerplay/hwmgr/ppatomfwctrl.c:78: warning: Function parameter or member 'hwmgr' not described in 'pp_atomfwctrl_is_voltage_controlled_by_gpio_v4'
 drivers/gpu/drm/amd/amdgpu/../pm/powerplay/hwmgr/ppatomfwctrl.c:78: warning: Function parameter or member 'voltage_type' not described in 'pp_atomfwctrl_is_voltage_controlled_by_gpio_v4'
 drivers/gpu/drm/amd/amdgpu/../pm/powerplay/hwmgr/ppatomfwctrl.c:78: warning: Function parameter or member 'voltage_mode' not described in 'pp_atomfwctrl_is_voltage_controlled_by_gpio_v4'
 drivers/gpu/drm/amd/amdgpu/../pm/powerplay/hwmgr/ppatomfwctrl.c:211: warning: Function parameter or member 'hwmgr' not described in 'pp_atomfwctrl_get_pp_assign_pin'
 drivers/gpu/drm/amd/amdgpu/../pm/powerplay/hwmgr/ppatomfwctrl.c:211: warning: Function parameter or member 'pin_id' not described in 'pp_atomfwctrl_get_pp_assign_pin'
 drivers/gpu/drm/amd/amdgpu/../pm/powerplay/hwmgr/ppatomfwctrl.c:211: warning: Function parameter or member 'gpio_pin_assignment' not described in 'pp_atomfwctrl_get_pp_assign_pin'
 drivers/gpu/drm/amd/amdgpu/../pm/powerplay/hwmgr/ppatomfwctrl.c:232: warning: Function parameter or member 'hwmgr' not described in 'pp_atomfwctrl_enter_self_refresh'

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>