drm/amd/amdgpu/gfx_v9_0: Make called-by-reference only function static

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

 drivers/gpu/drm/amd/amdgpu/gfx_v9_0.c:2998:6: warning: no previous prototype for ‘gfx_v9_0_rlc_stop’ [-Wmissing-prototypes]

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_v9_0.c

@@ -2993,7 +2993,7 @@ static void gfx_v9_0_init_pg(struct amdgpu_device *adev)
 	}
 }
 
-void gfx_v9_0_rlc_stop(struct amdgpu_device *adev)
+static void gfx_v9_0_rlc_stop(struct amdgpu_device *adev)
 {
 	WREG32_FIELD15(GC, 0, RLC_CNTL, RLC_ENABLE_F32, 0);
 	gfx_v9_0_enable_gui_idle_interrupt(adev, false);