dmaengine: imx-sdma: switch to dynamic context mode after script loaded

Below comments got from Page4724 of Reference Manual of i.mx6q:
http://cache.freescale.com/files/32bit/doc/ref_manual/IMX6DQRM.pdf



--"Static context mode should be used for the first channel called
after reset to ensure that the all context RAM for that channel is
initialized during the context SAVE phase when the channel is
done or yields. Subsequent calls to the same channel or
different channels may use any of the dynamic context modes.
This will ensure that all context locations for the bootload
channel are initialized, and prevent undefined values in context
RAM from being loaded during the context restore if the
channel is re-started later"

Unfortunately, the rule was broken by commit(5b28aa31)
.This patch just take them back.

Signed-off-by: Robin Gong <b38343@freescale.com>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>