gpu: Stop using TheISA in the GPU TLB.

gpu: Stop using TheISA in the GPU TLB.

This class is defined inside the X86ISA namespace, so there's no point
in pretending it's generic. Remove TheISA and let the code access what
it needs from X86ISA naturally since it's there already.

Change-Id: I21b5d2d2b9af6aa0c10ddbb5b3ddca1692188dcc
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/34173
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Matthew Poremba <matthew.poremba@amd.com>
Reviewed-by: Matt Sinclair <mattdsinclair@gmail.com>
Maintainer: Matt Sinclair <mattdsinclair@gmail.com>