media: venus: Mark bufreq_enc with static keyword

Fix the following sparse warning:

hfi_plat_bufs_v6.c:1242:5: warning: symbol 'bufreq_enc' was not declared.
 Should it be static?

Signed-off-by: Zou Wei <zou_wei@huawei.com>
Signed-off-by: Stanimir Varbanov <stanimir.varbanov@linaro.org>