MGS-1923 [#imx-19] use device mutex to sync command operations

MGS-1923 [#imx-19] use device mutex to sync command operations

kernel mutex cannot be shared with gpu0,gpu1 and bridge mode together.
use device mutex to prevent the sync problem for gpu command operations.

this change is reasonable and fixed one hole in existing implementation.
but this patch cannot fix the GPU hang for MGS-2429.

Date: Nov 28, 2016
Signed-off-by: Xianzhong <xianzhong.li@nxp.com>