gpu-compute: use X86ISA::TlbEntry over GpuTlbEntry

gpu-compute: use X86ISA::TlbEntry over GpuTlbEntry

GpuTlbEntry was derived from a vanilla X86ISA::TlbEntry definition. It
wrapped the class and included an extra member "valid". This member was
intended to report on the validity of the entry, however it introduced
bugs when folks forgot to set field properly in the code. So, instead of
keeping the extra field which we might forget to set, we track validity by
using nullptr for invalid tlb entries (as the tlb entries are dynamically
allocated). This saves on the extra class definition and prevents bugs
creeping into the code since the checks are intrinsically tied into
accessing any of the X86ISA::TlbEntry members.

This changeset fixes the issues introduced by a8d030522, a4e722725, and
2a15bfd79.

Change-Id: I30ebe3ec223fb833f3795bf0403d0016ac9a8bc2
Reviewed-on: https://gem5-review.googlesource.com/10481
Reviewed-by: Brandon Potter <Brandon.Potter@amd.com>
Maintainer: Anthony Gutierrez <anthony.gutierrez@amd.com>