drm/amdgpu/vcn: fix no previous prototype warning

Declare 'static', as the function is not intended to be used
outside of this translation unit.

Fixes: 4ed49c95 ("drm/amdgpu/vcn: add unified queue ib test")
Reported-by: kernel test robot <lkp@intel.com>
Reviewed-by: James Zhu <James.Zhu@amd.com>
Signed-off-by: Ruijing Dong <ruijing.dong@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

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

@@ -330,7 +330,7 @@ int amdgpu_vcn_sw_fini(struct amdgpu_device *adev)
 }
 
 /* from vcn4 and above, only unified queue is used */
-bool amdgpu_vcn_using_unified_queue(struct amdgpu_ring *ring)
+static bool amdgpu_vcn_using_unified_queue(struct amdgpu_ring *ring)
 {
 	struct amdgpu_device *adev = ring->adev;
 	bool ret = false;