drm/i915: Track pinned VMA

Treat the VMA as the primary struct responsible for tracking bindings
into the GPU's VM. That is we want to treat the VMA returned after we
pin an object into the VM as the cookie we hold and eventually release
when unpinning. Doing so eliminates the ambiguity in pinning the object
and then searching for the relevant pin later.

v2: Joonas' stylistic nitpicks, a fun rebase.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1471254551-25805-27-git-send-email-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/intel_fbc.c

@@ -737,7 +737,7 @@ static void intel_fbc_update_state_cache(struct intel_crtc *crtc,
 	/* FIXME: We lack the proper locking here, so only run this on the
 	 * platforms that need. */
 	if (IS_GEN(dev_priv, 5, 6))
-		cache->fb.ilk_ggtt_offset = i915_gem_obj_ggtt_offset(obj);
+		cache->fb.ilk_ggtt_offset = i915_gem_object_ggtt_offset(obj, NULL);
 	cache->fb.pixel_format = fb->pixel_format;
 	cache->fb.stride = fb->pitches[0];
 	cache->fb.fence_reg = obj->fence_reg;