drm/amd/display/amdgpu_dm/amdgpu_dm_color: Demote a misuse and fix another kernel...

drm/amd/display/amdgpu_dm/amdgpu_dm_color: Demote a misuse and fix another kernel-doc header

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

 drivers/gpu/drm/amd/amdgpu/../display/amdgpu_dm/amdgpu_dm_color.c:128: warning: Function parameter or member 'lut' not described in '__drm_lut_to_dc_gamma'
 drivers/gpu/drm/amd/amdgpu/../display/amdgpu_dm/amdgpu_dm_color.c:128: warning: Function parameter or member 'gamma' not described in '__drm_lut_to_dc_gamma'
 drivers/gpu/drm/amd/amdgpu/../display/amdgpu_dm/amdgpu_dm_color.c:128: warning: Function parameter or member 'is_legacy' not described in '__drm_lut_to_dc_gamma'
 drivers/gpu/drm/amd/amdgpu/../display/amdgpu_dm/amdgpu_dm_color.c:426: warning: Function parameter or member 'dc_plane_state' not described in 'amdgpu_dm_update_plane_color_mgmt'

Cc: Harry Wentland <harry.wentland@amd.com>
Cc: Leo Li <sunpeng.li@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>