drm/amd/amdgpu/amdgpu_virt: Make local function 'amdgpu_virt_update_vf2pf_work_item()' static

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

 drivers/gpu/drm/amd/amdgpu/amdgpu_virt.c:560:6: warning: no previous prototype for ‘amdgpu_virt_update_vf2pf_work_item’ [-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: 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/amdgpu/amdgpu_virt.c

@@ -555,7 +555,7 @@ static int amdgpu_virt_write_vf2pf_data(struct amdgpu_device *adev)
 	return 0;
 }
 
-void amdgpu_virt_update_vf2pf_work_item(struct work_struct *work)
+static void amdgpu_virt_update_vf2pf_work_item(struct work_struct *work)
 {
 	struct amdgpu_device *adev = container_of(work, struct amdgpu_device, virt.vf2pf_work.work);