drm/i915: Do not leak objects after capturing error state

While running kmemleak chasing a different memleak, I saw that the
capture_error_state function was leaking some objects, for example:

unreferenced object 0xffff8800a9b72148 (size 8192):
  comm "kworker/u16:0", pid 1499, jiffies 4295201243 (age 990.096s)
  hex dump (first 32 bytes):
    00 00 04 00 00 00 00 00 5d f4 ff ff 00 00 00 00  ........].......
    00 30 b0 01 00 00 00 00 37 00 00 00 00 00 00 00  .0......7.......
  backtrace:
    [<ffffffff811e5ae4>] create_object+0x104/0x2c0
    [<ffffffff8178f50a>] kmemleak_alloc+0x7a/0xc0
    [<ffffffff811cde4b>] __kmalloc+0xeb/0x220
    [<ffffffffa038f1d9>] kcalloc.constprop.12+0x2d/0x2f [i915]
    [<ffffffffa0316064>] i915_capture_error_state+0x3f4/0x1660 [i915]
    [<ffffffffa03207df>] i915_handle_error+0x7f/0x660 [i915]
    [<ffffffffa03210f7>] i915_hangcheck_elapsed+0x2e7/0x470 [i915]
    [<ffffffff8108d574>] process_one_work+0x144/0x490
    [<ffffffff8108dfbd>] worker_thread+0x11d/0x530
    [<ffffffff81094079>] kthread+0xc9/0xe0
    [<ffffffff817a2398>] ret_from_fork+0x58/0x90
    [<ffffffffffffffff>] 0xffffffffffffffff

The following objects are allocated in i915_gem_capture_buffers, but not
released in i915_error_state_free:
  - error->active_bo_count
  - error->pinned_bo
  - error->pinned_bo_count
  - error->active_bo[vm_count] (allocated in i915_gem_capture_vm).

The leaks were introduced by
commit 95f5301d
Author: Ben Widawsky <ben@bwidawsk.net>
Date:   Wed Jul 31 17:00:15 2013 -0700

    drm/i915: Update error capture for VMs

v2: Reuse iterator and add culprit commit details (Chris)

Cc: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Michel Thierry <michel.thierry@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>