drm/amdkfd: Add unmap from GPU SMI event

SVM range unmapped from GPUs when range is unmapped from CPU, or with
xnack on from MMU notifier when range is evicted or migrated.

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>