fix missing mutex cleanup

Signed-off-by: Carlos Rafael Giani <dv@pseudoterminal.org>
Fix in available framebuffers counter

Sometimes, the counter must not be decremented, even though a frame was
produced. This is the case when VPU_DEC_OUTPUT_DIS is also set.

Signed-off-by: Carlos Rafael Giani <dv@pseudoterminal.org>