drm/i915: use default 200 Hz backlight frequency

If the backlight modulation frequency can't be extracted from the
registers or from VBT, use 200 Hz as the default. This may enable
backlight on some machines that previously failed.

Reviewed-by: Imre Deak <imre.deak@intel.com>
Tested-by: Imre Deak <imre.deak@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1448958232-26520-2-git-send-email-jani.nikula@intel.com