MGS-2857 [#imx-530] fix spinlock in gpu kernel driver

MGS-2857 [#imx-530] fix spinlock in gpu kernel driver

there are two critical problems with spinlock in gpu driver:
1. spin_unlock is missing when pte_offset_map_lock return invalid pte pointer,
this will cause dead lock in stress case.
2. pte_unmap is missing in spinlock debug build, this will cause kernel panic,
which can be reproduced when enable CONFIG_DEBUG_SPINLOCK in kernel build.

Date: May 02, 2017
Signed-off-by: Xianzhong <xianzhong.li@nxp.com>