drm_gralloc: fix random crash with wildpointer

two drm handle may use same bo, but there is no
reference protect. if one of the drm handle release
the bo, another handle's bo become a wildpointer,
any read/write on the wildpointer will cause system
unstable, crash.

Change-Id: Ieaca522e3372dba82c48961499b9b657ca33cd15
Signed-off-by: Mark Yao <mark.yao@rock-chips.com>