drm/i915/backlight: prefer dev_priv over dev pointer

Use dev_priv rather than dev pointer where applicable. Remove plenty of
unnecessary temp variables. No functional changes.
Reviewed-by: Mika Kuoppala <mika.kuoppala@intel.com>
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/1450428695-28831-1-git-send-email-jani.nikula@intel.com