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>