drm/i915: Switch eviction code to use vmas

[Dan Carpenter]

Hello Ben Widawsky,

Here is another use after free warning.  It's some new static checker
stuff that I haven't pushed because it has lots of false postives.

The patch f7795b1d0b47: "drm/i915: Switch eviction code to use vmas" 
from Aug 14, 2013, leads to the following warning:
"drivers/gpu/drm/i915/i915_gem_evict.c:145 i915_gem_evict_something()
	 warn: 'vma' was already freed."


drivers/gpu/drm/i915/i915_gem_evict.c
   137          /* Unbinding will emit any required flushes */
   138          while (!list_empty(&eviction_list)) {
   139                  vma = list_first_entry(&eviction_list,
   140                                         struct i915_vma,
   141                                         exec_list);
   142                  if (ret == 0)
   143                          ret = i915_vma_unbind(vma);
                                      ^^^^^^^^^^^^^^^^^^^^
This often frees the "vma".

   144  
   145                  list_del_init(&vma->exec_list);
                                      ^^^^^^^^^^^^^^^
Dereference.

   146                  drm_gem_object_unreference(&vma->obj->base);
   147          }

regards,
dan carpenter