drm/rockchip: fix mm_dump NULL pointer crash

drm/rockchip: fix mm_dump NULL pointer crash

drm mm is not initial on non-iommu context.

Fix crash:
[   11.635376] Unable to handle kernel NULL pointer dereference at virtual address 00000000
[   11.636148] pgd = ffffffc0de19d000
[   11.636474] [00000000] *pgd=0000000000000000, *pud=0000000000000000
[   11.637088] Internal error: Oops: 96000045 [#1] PREEMPT SMP
[   11.637594] Modules linked in:
[   11.637902] CPU: 4 PID: 424 Comm: sh Not tainted 4.4.71 #146
[   11.638406] Hardware name: Rockchip RK3399 Evaluation Board v3 (Android) (DT)
[   11.639050] task: ffffffc0de796800 ti: ffffffc0de260000 task.ti: ffffffc0de260000
[   11.639736] PC is at __mutex_lock_slowpath+0xa8/0x178
[   11.640198] LR is at __mutex_lock_slowpath+0x7c/0x178
[   11.788003] [<ffffff8008b85b5c>] __mutex_lock_slowpath+0xa8/0x178
[   11.788560] [<ffffff8008b85c58>] mutex_lock+0x2c/0x44
[   11.789024] [<ffffff800849e8d8>] rockchip_drm_mm_dump+0x30/0x5c
[   11.789560] [<ffffff80081d612c>] seq_read+0x1a8/0x3f8
[   11.790017] [<ffffff80081b4208>] __vfs_read+0x38/0xf4
[   11.790472] [<ffffff80081b4adc>] vfs_read+0x84/0x128
[   11.790927] [<ffffff80081b552c>] SyS_read+0x54/0xa4
[   11.791372] [<ffffff80080828b0>] el0_svc_naked+0x24/0x28

Change-Id: I342311da43d50b743257b81425dfc6a327306803
Signed-off-by: Mark Yao <mark.yao@rock-chips.com>