drm/amdkfd: handle VMA remove race

VMA may be removed before unmap notifier callback, and deferred list
work remove range, return success for this special case as we are
handling stale retry fault.

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>