drm/i915: Avoid unmapping pages from a NULL address space

Found by gem_stress.

As we perform retirement from a workqueue, it is possible for us to free
and unbind objects after the last close on the device, and so after the
address space has been torn down and reset to NULL:

BUG: unable to handle kernel NULL pointer dereference at 00000054
IP: [<c1295a20>] mutex_lock+0xf/0x27
*pde = 00000000
Oops: 0002 [#1] SMP
last sysfs file: /sys/module/vt/parameters/default_utf8

Pid: 5, comm: kworker/u:0 Not tainted 2.6.38+ #214
EIP: 0060:[<c1295a20>] EFLAGS: 00010206 CPU: 1
EIP is at mutex_lock+0xf/0x27
EAX: 00000054 EBX: 00000054 ECX: 00000000 EDX: 00012fff
ESI: 00000028 EDI: 00000000 EBP: f706fe20 ESP: f706fe18
 DS: 007b ES: 007b FS: 00d8 GS: 0000 SS: 0068
Process kworker/u:0 (pid: 5, ti=f706e000 task=f7060d00 task.ti=f706e000)
Stack:
 f5aa3c60 00000000 f706fe74 c107e7df 00000246 dea55380 00000054 f5aa3c60
 f706fe44 00000061 f70b4000 c13fff84 00000008 f706fe54 00000000 00000000
 00012f00 00012fff 00000028 c109e575 f6b36700 00100000 00000000 f706fe90
Call Trace:
 [<c107e7df>] unmap_mapping_range+0x7d/0x1e6
 [<c109e575>] ? mntput_no_expire+0x52/0xb6
 [<c11c12f6>] i915_gem_release_mmap+0x49/0x58
 [<c11c3449>] i915_gem_object_unbind+0x4c/0x125
 [<c11c353f>] i915_gem_free_object_tail+0x1d/0xdb
 [<c11c55a2>] i915_gem_free_object+0x3d/0x41
 [<c11a6be2>] ? drm_gem_object_free+0x0/0x27
 [<c11a6c07>] drm_gem_object_free+0x25/0x27
 [<c113c3ca>] kref_put+0x39/0x42
 [<c11c0a59>] drm_gem_object_unreference+0x16/0x18
 [<c11c0b15>] i915_gem_object_move_to_inactive+0xba/0xbe
 [<c11c0c87>] i915_gem_retire_requests_ring+0x16e/0x1a5
 [<c11c3645>] i915_gem_retire_requests+0x48/0x63
 [<c11c36ac>] i915_gem_retire_work_handler+0x4c/0x117
 [<c10385d1>] process_one_work+0x140/0x21b
 [<c103734c>] ? __need_more_worker+0x13/0x2a
 [<c10373b1>] ? need_to_create_worker+0x1c/0x35
 [<c11c3660>] ? i915_gem_retire_work_handler+0x0/0x117
 [<c1038faf>] worker_thread+0xd4/0x14b
 [<c1038edb>] ? worker_thread+0x0/0x14b
 [<c103be1b>] kthread+0x68/0x6d
 [<c103bdb3>] ? kthread+0x0/0x6d
 [<c12970f6>] kernel_thread_helper+0x6/0x10
Code: 00 e8 98 fe ff ff 5d c3 55 89 e5 3e 8d 74 26 00 ba 01 00 00 00 e8
84 fe ff ff 5d c3 55 89 e5 53 8d 64 24 fc 3e 8d 74 26 00 89 c3 <f0> ff
08 79 05 e8 ab ff ff ff 89 e0 25 00 e0 ff ff 89 43 10 58
EIP: [<c1295a20>] mutex_lock+0xf/0x27 SS:ESP 0068:f706fe18
CR2: 0000000000000054

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Keith Packard <keithp@keithp.com>