drm/i915/gem: Avoid iterating an empty list

Our __sgt_iter assumes that the scattergather list has at least one
element. But during construction we may fail in allocating the first
page, and so mark the first element as the terminator. This is
unexpected!

[22555.524752] RIP: 0010:shmem_get_pages+0x506/0x710 [i915]
[22555.524759] Code: 49 8b 2c 24 31 c0 66 89 44 24 40 48 85 ed 0f 84 62 01 00 00 4c 8b 75 00 8b 5d 08 44 8b 7d 0c 48 8b 0d 7e 34 07 e2 49 83 e6 fc <49> 8b 16 41 01 df 48 89 cf 48 89 d0 48 c1 e8 2d 48 85 c9 0f 84 c8
[22555.524765] RSP: 0018:ffffc9000053f9d0 EFLAGS: 00010246
[22555.524770] RAX: 0000000000000000 RBX: 0000000000000000 RCX: ffff8881ffffa000
[22555.524774] RDX: fffffffffffffff4 RSI: ffffffffffffffff RDI: ffffffff821efe00
[22555.524778] RBP: ffff8881b099ab00 R08: 0000000000000000 R09: 00000000fffffff4
[22555.524782] R10: 0000000000000002 R11: 00000000ffec0a02 R12: ffff8881cd3c8d60
[22555.524786] R13: 00000000fffffff4 R14: 0000000000000000 R15: 0000000000000000
[22555.524790] FS:  00007f4fbeb9b9c0(0000) GS:ffff8881f8580000(0000) knlGS:0000000000000000
[22555.524795] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[22555.524799] CR2: 0000000000000000 CR3: 00000001ec7f0004 CR4: 00000000001606e0
[22555.524803] Call Trace:
[22555.524919]  __i915_gem_object_get_pages+0x4f/0x60 [i915]

Fixes: 85d1225e

 ("drm/i915: Introduce & use new lightweight SGL iterators")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Matthew Auld <matthew.auld@intel.com>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Cc: <stable@vger.kernel.org> # v4.8+
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Reviewed-by: Maciej Patelczyk <maciej.patelczyk@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200522132706.5133-1-chris@chris-wilson.co.uk