drm/i915/bios: prefer using dev_priv over dev pointer

dev_priv is the new black. Or something.

Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1450271061-32646-3-git-send-email-jani.nikula@intel.com