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>

drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm_color.c

@@ -119,7 +119,7 @@ static bool __is_lut_linear(const struct drm_color_lut *lut, uint32_t size)
 	return true;
 }
 
-/**
+/*
  * Convert the drm_color_lut to dc_gamma. The conversion depends on the size
  * of the lut - whether or not it's legacy.
  */
@@ -413,7 +413,7 @@ int amdgpu_dm_update_crtc_color_mgmt(struct dm_crtc_state *crtc)
 /**
  * amdgpu_dm_update_plane_color_mgmt: Maps DRM color management to DC plane.
  * @crtc: amdgpu_dm crtc state
- * @ dc_plane_state: target DC surface
+ * @dc_plane_state: target DC surface
  *
  * Update the underlying dc_stream_state's input transfer function (ITF) in
  * preparation for hardware commit. The transfer function used depends on