drm/amd/amdgpu/gfx_v10_0: Demote kernel-doc abuse

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

 drivers/gpu/drm/amd/amdgpu/gfx_v10_0.c:51: warning: This comment starts with '/**', but isn't a kernel-doc comment. Refer Documentation/doc-guide/kernel-doc.rst

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: Sumit Semwal <sumit.semwal@linaro.org>
Cc: amd-gfx@lists.freedesktop.org
Cc: dri-devel@lists.freedesktop.org
Cc: linux-media@vger.kernel.org
Cc: linaro-mm-sig@lists.linaro.org
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/amdgpu/gfx_v10_0.c

@@ -47,7 +47,7 @@
 #include "gfx_v10_0.h"
 #include "nbio_v2_3.h"
 
-/**
+/*
  * Navi10 has two graphic rings to share each graphic pipe.
  * 1. Primary ring
  * 2. Async ring