drm/i915: Convert backlight_lock to a mutex

Originally the irq safe spinlock was required because of asle
interrupts. But since

commit 91a60f20
Author: Jani Nikula <jani.nikula@intel.com>
Date:   Thu Oct 31 18:55:48 2013 +0200

    drm/i915: move opregion asle request handling to a work queue

there's no need for this any more. So switch to the simpler mutex.

v2: Cite the right commit, spotted by Jani.

Cc: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>