i915: correctly handling failed allocation

Since devm_kzalloc can be failed, it needs to be checked
if not, NULL dereference could be happened.

Signed-off-by: Insu Yun <wuninsu@gmail.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1451491169-35068-1-git-send-email-wuninsu@gmail.com
Signed-off-by: Jani Nikula <jani.nikula@intel.com>