drm/vc4: mark vc4_bo_cache_purge() static

We get 1 warning when building kernel with W=1:
drivers/gpu/drm/vc4/vc4_bo.c:147:6: warning: no previous prototype for 'vc4_bo_cache_purge' [-Wmissing-prototypes]

In fact, this function is only used in the file in which it is
declared and don't need a declaration, but can be made static.
So this patch marks it 'static'.

Signed-off-by: Baoyou Xie <baoyou.xie@linaro.org>
Signed-off-by: Eric Anholt <eric@anholt.net>