Two patches to improve gang submit with reserved VMIDs

[Christian König]

Hi guys,

those two patches clean up gang submit. The first one should prevent
crashes when gang submit is used together with a reserved VMID.

The second rejects gang submits with a reserved VMID when this won't
work because of HW limitations.

Only smoke tested since I need more HW for this setup

@Vitaly: If you have some time please test them with your stand alone
gang submit test first and then we need to come up with a combined
test case for gang submit with reserved VMID.

Regards,
Christian.