dmaengine: imx-sdma: use GFP_NOWAIT for dma descriptor allocations

commit 64068853bc77786d1a28abb4087d6a3e93aedbe2 upstream.

DMA buffer descriptors aren't allocated from atomic context, so they
can use the less heavyweigth GFP_NOWAIT.

Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Signed-off-by: Robin Gong <yibin.gong@nxp.com>
Cc: stable <stable@vger.kernel.org>
Signed-off-by: Vinod Koul <vkoul@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>