dmaengine: imx-sdma: refine to load context only once

The context loaded only one time before channel running,but
currently sdma_config_channel() and dma_prep_* duplicated with
sdma_load_context(), so refine it to load context only one time
before channel running and reload after the channel terminated.

Signed-off-by: Robin Gong <yibin.gong@nxp.com>
Signed-off-by: Vinod Koul <vkoul@kernel.org>