drm/i915: Check current i915_vma.pin_count status first on unbind

Do an early rejection of a i915_vma_unbind() attempt if the i915_vma is
currently pinned, without waiting to see if the inflight operations may
unpin it. We see this problem with the shrinker trying to unbind the
active vma from inside its bind worker:

<6> [472.618968] Workqueue: events_unbound fence_work [i915]
<4> [472.618970] Call Trace:
<4> [472.618974]  ? __schedule+0x2e5/0x810
<4> [472.618978]  schedule+0x37/0xe0
<4> [472.618982]  schedule_preempt_disabled+0xf/0x20
<4> [472.618984]  __mutex_lock+0x281/0x9c0
<4> [472.618987]  ? mark_held_locks+0x49/0x70
<4> [472.618989]  ? _raw_spin_unlock_irqrestore+0x47/0x60
<4> [472.619038]  ? i915_vma_unbind+0xae/0x110 [i915]
<4> [472.619084]  ? i915_vma_unbind+0xae/0x110 [i915]
<4> [472.619122]  i915_vma_unbind+0xae/0x110 [i915]
<4> [472.619165]  i915_gem_object_unbind+0x1dc/0x400 [i915]
<4> [472.619208]  i915_gem_shrink+0x328/0x660 [i915]
<4> [472.619250]  ? i915_gem_shrink_all+0x38/0x60 [i915]
<4> [472.619282]  i915_gem_shrink_all+0x38/0x60 [i915]
<4> [472.619325]  vm_alloc_page.constprop.25+0x1aa/0x240 [i915]
<4> [472.619330]  ? rcu_read_lock_sched_held+0x4d/0x80
<4> [472.619363]  ? __alloc_pd+0xb/0x30 [i915]
<4> [472.619366]  ? module_assert_mutex_or_preempt+0xf/0x30
<4> [472.619368]  ? __module_address+0x23/0xe0
<4> [472.619371]  ? is_module_address+0x26/0x40
<4> [472.619374]  ? static_obj+0x34/0x50
<4> [472.619376]  ? lockdep_init_map+0x4d/0x1e0
<4> [472.619407]  setup_page_dma+0xd/0x90 [i915]
<4> [472.619437]  alloc_pd+0x29/0x50 [i915]
<4> [472.619470]  __gen8_ppgtt_alloc+0x443/0x6b0 [i915]
<4> [472.619503]  gen8_ppgtt_alloc+0xd7/0x300 [i915]
<4> [472.619535]  ppgtt_bind_vma+0x2a/0xe0 [i915]
<4> [472.619577]  __vma_bind+0x26/0x40 [i915]
<4> [472.619611]  fence_work+0x1c/0x90 [i915]
<4> [472.619617]  process_one_work+0x26a/0x620

Fixes: 2850748e ("drm/i915: Pull i915_vma_pin under the vm->mutex")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200403120150.17091-1-chris@chris-wilson.co.uk