drm/amdkfd: flush TLB after updating GPU page table

To workaround the situation that vm retry fault keep coming after page
table update. We are investigating the root cause, but once this issue
happens, application will stuck and sometimes have to reboot to recover.

Signed-off-by: Philip Yang <Philip.Yang@amd.com>
Reviewed-by: Felix Kuehling <Felix.Kuehling@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>