[omx] Restrict the number of outstanding jpeg calls to gpu

Allowing more than one outstanding call to gpu for texture encode/decode can be beneficial as processing
can be overlapped with vchiq message transfer.

Experimentally, 3 outstanding operations is optimal.

Allowing more that this ties up memory on GPU without any performance benefit, so avoid submitting more jobs
If i understand it correct, a local g_count_val would mean no global limit to three.