MGS-374 [#1500] fix GPU kernel debugfs crash issue

this crash issue is caused by kernel NULL pointer when access GPU database,
GPU database is shared by all kernels, it can be queried with any valid kernel.
this patch will find the valid kernel pointer to avoid GPU kernel crash.

the crash backtrace with 'cat /sys/kernel/debug/gc/vidmem' on i.mx6sl:

[<80480600>] (gckKERNEL_FindDatabase+0x8/0xec) from [<80478db0>] (vidmem_show+0x2c/0x60)
[<80478db0>] (vidmem_show+0x2c/0x60) from [<800e4d5c>] (seq_read+0x1dc/0x47c)
[<800e4d5c>] (seq_read+0x1dc/0x47c) from [<800c7164>] (vfs_read+0x98/0x144)
[<800c7164>] (vfs_read+0x98/0x144) from [<800c77c4>] (SyS_read+0x3c/0x78)
[<800c77c4>] (SyS_read+0x3c/0x78) from [<8000e080>] (ret_fast_syscall+0x0/0x30)

Date: Dec 18, 2014
Signed-off-by: Xianzhong <b07117@freescale.com>
Acked-by: Jason Liu
(cherry picked from commit 5b59e9d67ee1ce25e43fcb8934fcf0c75abfd0e3)