gpu-compute: fix bad asserts in gpu tlb and cu tlb port

gpu-compute: fix bad asserts in gpu tlb and cu tlb port

change 2a15bfd79ced20a6d4cbf0a0a4c2fbb1444b9a44 introduced
a few bugs in the tlb of the cu. asserts in the gpu tlb
and cu expected the page table lookup() function to return
a bool, and this value was used directly in the gpu tlb's
assert and it was kept in the gpu tlb entry, where later
the cu would assert that it is true.

this change fixes the issue by checking the validity of
the pte pointer returned by lookup() in order to set
the validity of the tlb entry itself.

Change-Id: Ief1f205db65f1911fd132acd314e4407c5e3ffdf
Reviewed-on: https://gem5-review.googlesource.com/10001
Maintainer: Anthony Gutierrez <anthony.gutierrez@amd.com>
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>