media: venus: vdec: Fix forgotten mutex unlock in start streaming

This fixes the following smatch warning in the error path:

drivers/media/platform/qcom/venus/vdec.c:968 vdec_start_streaming()
 warn: inconsistent returns 'mutex:&inst->lock'.
  Locked on:   line 952
  Unlocked on: line 963
               line 968

by goto mutex unlock.

Signed-off-by: Stanimir Varbanov <stanimir.varbanov@linaro.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>