drm/amdkfd: Optimize out some duplicated code in kfd_signal_iommu_event()

memory_exception_data is already initialized for not-present faults.
It only needs to be overridden for permission faults.

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>