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