gpu-compute: Fix deadlock in fetch_unit after branch instruction

gpu-compute: Fix deadlock in fetch_unit after branch instruction

The following deadlock was occuring in fetch_unit w/timingSim:
1. exec() is called, a wave is ready to fetch, so it sets pendingFetch
2. A packet is sent to ITLB to fetch for that wave
3. The wave executes a branch, causing the fetch buffer to be cleared
4. The packet is handled, and fetch() is called. However, because the
fetch buffer was cleared, it returns doing nothing.
5. exec() gets called again, but the wave will never be scheduled to
fetch, as pendingFetch is still set to true.

This patch clears pendingFetch (and dropFetch) before returning in fetch()
when the fetch buffer has been cleared.

dropFetch needed to be cleared otherwise gem5 would crash.

Change-Id: Iccbac7defc4849c19e8b17aa2492da641defb772
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/34555
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Reviewed-by: Matt Sinclair <mattdsinclair@gmail.com>
Reviewed-by: Anthony Gutierrez <anthony.gutierrez@amd.com>
Maintainer: Anthony Gutierrez <anthony.gutierrez@amd.com>
Tested-by: kokoro <noreply+kokoro@google.com>