MA-10665-3 [#imx-615] Fix monkey test reboot issue due to drm gem bug

MA-10665-3 [#imx-615] Fix monkey test reboot issue due to drm gem bug

Fix monkey test reboot issue due to drm gem bug.
1. deref gem_obj in the end of funtion
2. replace async unlock with bottom half unlock.

Monkey commands:
while true;do monkey --pct-syskeys 0 -v 100000;done

Reboot log:
[ 884.062236] WARNING: CPU: 3 PID: 3987 at drivers/gpu/drm/drm_gem.c:776 drm_gem_object_release+0x40/0x48
[ 884.076072] Modules linked in: bcmdhd
[ 884.079760]
[ 884.081260] CPU: 3 PID: 3987 Comm: ReferenceQueueD Not tainted 4.9.56-00115-ge9a7131 #1
[ 884.089268] Hardware name: Freescale i.MX8QXP MEK (DT)
[ 884.094410] task: ffff80004cf6de80 task.stack: ffff800839838000
[ 884.100338] PC is at drm_gem_object_release+0x40/0x48
[ 884.105405] LR is at viv_gem_free_object+0x64/0x90

Date: Nov 27, 2017
Signed-off-by: Richard Liu <xuegang.liu@nxp.com>