drm/i915: Remove a couple pointless WARN_ONs

Just two WARN_ONs followed by pointer dereference I spotted by accident.

v2: Remove some more of the same.

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: http://patchwork.freedesktop.org/patch/msgid/1461080770-14693-1-git-send-email-tvrtko.ursulin@linux.intel.com