Fix an oops in i915_gem_retire_requests()

dev_priv->hw_status_page can be NULL, if i915_gem_retire_requests()
is called from i915_gem_busy_ioctl().

Signed-off-by Karsten Wiese <fzu@wemgehoertderstaat.de>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>