gpu-compute: Fix Y-dimension ABI decode

gpu-compute: Fix Y-dimension ABI decode

We currently have a bug in decoding workitem ID from the kernel
descriptor with multiple dimensions.  The enable_vgpr_workitem_id bits
are currently seperated into x and y components, when they should be
treated as a single 2 bit value, where y is enabled when it is > 0,
and z is enabled when it is > 1.  The current setup allows a kernel
launch with vgprs reserved for the z dimension and not the y dimension,
which is incorrect.

Change-Id: Iee64b207feb95bcf064898d5db33b8f201e25323
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/29965
Maintainer: Anthony Gutierrez <anthony.gutierrez@amd.com>
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Matt Sinclair <mattdsinclair@gmail.com>