amdkfd: init aperture once per process

Since the user space may call open() more that once from the same process,
the aperture initialization should be moved from kfd_open()

Signed-off-by: Alexey Skidanov <Alexey.Skidanov@amd.com>
Reviewed-by: Oded Gabbay <oded.gabbay@amd.com>
Signed-off-by: Oded Gabbay <oded.gabbay@amd.com>