drm/amdkfd: Workaround to accommodate Raven too many PPR issue

On Raven multiple PPRs can be queued up by the hardware. When the
first of those requests is handled by the IOMMU driver, the memory
access succeeds. After that the application may be done with the
memory and unmap it. At that point the page table entries are
invalidated, but there are still outstanding duplicate PPRs for those
addresses. When the IOMMU driver processes those duplicate requests,
it finds invalid page table entries and triggers an invalid PPR fault.

As a workaround, don't signal invalid PPR faults on Raven to avoid
segfaulting applications that haven't done anything wrong. As a side
effect, real GPU memory access faults may go unnoticed by the
application.

Signed-off-by: Yong Zhao <yong.zhao@amd.com>
Reviewed-by: Felix Kuehling <Felix.Kuehling@amd.com>
Signed-off-by: Felix Kuehling <Felix.Kuehling@amd.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Oded Gabbay <oded.gabbay@gmail.com>