drm/amd/display: fix doc entry for bb_from_dmub

Fixes the warning:

Function parameter or struct member 'bb_from_dmub' not described in 'amdgpu_display_manager'

Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Aurabindo Pillai <aurabindo.pillai@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

Author: Aurabindo Pillai

drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.h

@@ -587,7 +587,9 @@ struct amdgpu_display_manager {
 	 */
 	struct mutex dpia_aux_lock;
 
-	/*
+	/**
+	 * @bb_from_dmub:
+	 *
 	 * Bounding box data read from dmub during early initialization for DCN4+
 	 */
 	struct dml2_soc_bb *bb_from_dmub;