If the NOP count of an SDMA NOP packet goes beyond the wptr address, the
queue decode method will loop infinitely. If a packet comes in with a
bad count this causes gem5 to hang. This change advances the rptr one
dword at a time until either reaching the NOP count or when rptr == wptr
to prevent this issue.
Change-Id: Ib2c0f74a477bff27890c9c064bb4190e76e513bd
7f71477f15