drm/amdkfd: lower the VAs base offset to 8KB

drm/amdkfd: lower the VAs base offset to 8KB

The low 16MB of virtual address space are currently reserved for kernel
mode allocations mapped into user virtual address space. This causes
conflicts with HMM/SVM mappings at low virtual addresses. We tried to
move those kernel mode allocations to the upper half of the 64-bit
virtual address space for GFX9, which is naturally reserved for kernel
use. However, TBA (trap handler code) has problems to access addresses
in the high virtual space. We have decided to set this to 8KB of the
lower address space as a temporary fix, while investigate TBA address
problem. It is very unlikely for user space to map memory at this low
region.

Signed-off-by: Alex Sierra <alex.sierra@amd.com>
Reviewed-by: Felix Kuehling <Felix.Kuehling@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>