drm/i915: Reduce a pin-leak BUG into a WARN

Pin-leaks persist and we get the perennial bug reports of machine
lockups to the BUG_ON(pin_count==MAX). If we instead loudly report that
the object cannot be pinned at that time it should prevent the driver from
locking up, and hopefully restore a semblance of working whilst still
leaving us a OOPS to debug.
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: stable@vger.kernel.org
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/i915_gem.c

@@ -3242,7 +3242,8 @@ i915_gem_object_pin(struct drm_i915_gem_object *obj,
 {
 	int ret;
 
-	BUG_ON(obj->pin_count == DRM_I915_GEM_OBJECT_MAX_PIN_COUNT);
+	if (WARN_ON(obj->pin_count == DRM_I915_GEM_OBJECT_MAX_PIN_COUNT))
+		return -EBUSY;
 
 	if (obj->gtt_space != NULL) {
 		if ((alignment && obj->gtt_offset & (alignment - 1)) ||