media: venus: provide ctx queue lock for ioctl synchronization

Video device has to provide a lock so that __video_do_ioctl()
can serialize IOCTL calls. Introduce a dedicated venus_inst
mutex for the purpose of vb2 operations synchronization.

Signed-off-by: Sergey Senozhatsky <senozhatsky@chromium.org>
Reviewed-by: Vikash Garodia <quic_vgarodia@quicinc.com>
Reviewed-by: Bryan O'Donoghue <bryan.odonoghue@linaro.org>
Signed-off-by: Stanimir Varbanov <stanimir.k.varbanov@gmail.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>