drm/vc4: make function vc4_allocate_bin_bo static

Sparse complains with following warning:
drivers/gpu/drm/vc4/vc4_v3d.c:222:1: warning: symbol
'vc4_allocate_bin_bo' was not declared. Should it be static?

Make vc4_allocate_bin static as it is not used outside of
vc4_v3d.c.
Signed-off-by: Vaishali Thakkar <vthakkar1994@gmail.com>
Signed-off-by: Eric Anholt <eric@anholt.net>
Link: https://patchwork.freedesktop.org/patch/msgid/20180425070953.17933-1-vthakkar1994@gmail.com

drivers/gpu/drm/vc4/vc4_v3d.c

@@ -218,8 +218,7 @@ int vc4_v3d_get_bin_slot(struct vc4_dev *vc4)
  * overall CMA pool before they make scenes complicated enough to run
  * out of bin space.
  */
-int
-vc4_allocate_bin_bo(struct drm_device *drm)
+static int vc4_allocate_bin_bo(struct drm_device *drm)
 {
 	struct vc4_dev *vc4 = to_vc4_dev(drm);
 	struct vc4_v3d *v3d = vc4->v3d;