drm/i915/debugfs: Show context objects in i915_gem_objects

One of the uses for i915_gem_objects is pin-pointing leaks. For this, we
can compare the number of allocated objects and who owns them, a
discrepancy here often indicates a kernel bug. One allocator of unreported
objects is for backing context objects, so include those in the listing.

v2: Take filelist_mutex which requires a little dance with struct_mutex
to avoid nesting filelist_mutex inside struct_mutex.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1464098023-3294-10-git-send-email-chris@chris-wilson.co.uk