drm/amd/pm/powerplay/hwmgr/vega10_hwmgr: Fix worthy function headers, demote barely documented one

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

 drivers/gpu/drm/amd/amdgpu/../pm/powerplay/hwmgr/vega10_hwmgr.c:1556: warning: Function parameter or member 'acg_freq' not described in 'vega10_populate_single_gfx_level'
 drivers/gpu/drm/amd/amdgpu/../pm/powerplay/hwmgr/vega10_hwmgr.c:1621: warning: Function parameter or member 'current_soc_did' not described in 'vega10_populate_single_soc_level'
 drivers/gpu/drm/amd/amdgpu/../pm/powerplay/hwmgr/vega10_hwmgr.c:1621: warning: Function parameter or member 'current_vol_index' not described in 'vega10_populate_single_soc_level'
 drivers/gpu/drm/amd/amdgpu/../pm/powerplay/hwmgr/vega10_hwmgr.c:1621: warning: Excess function parameter 'current_socclk_level' description in 'vega10_populate_single_soc_level'
 drivers/gpu/drm/amd/amdgpu/../pm/powerplay/hwmgr/vega10_hwmgr.c:1757: warning: Function parameter or member 'current_mem_vid' not described in 'vega10_populate_single_memory_level'
 drivers/gpu/drm/amd/amdgpu/../pm/powerplay/hwmgr/vega10_hwmgr.c:1757: warning: Function parameter or member 'current_memclk_level' not described in 'vega10_populate_single_memory_level'
 drivers/gpu/drm/amd/amdgpu/../pm/powerplay/hwmgr/vega10_hwmgr.c:1757: warning: Function parameter or member 'current_mem_soc_vind' not described in 'vega10_populate_single_memory_level'
 drivers/gpu/drm/amd/amdgpu/../pm/powerplay/hwmgr/vega10_hwmgr.c:2871: warning: Function parameter or member 'bitmap' not described in 'vega10_start_dpm'

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>