media: venus: hfi: Increase plat_buf_v6 o/p buffer count.

Presently, minimum buffer count for output buffers for h264 &
hevc codec is kept as 8 which might not be sufficient for
few use cases so increasing the value to 18.

Signed-off-by: Dikshita Agarwal <dikshita@codeaurora.org>
Signed-off-by: Bryan O'Donoghue <bryan.odonoghue@linaro.org>
Signed-off-by: Stanimir Varbanov <stanimir.varbanov@linaro.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>