drm/i915: don't hard code vlv backlight frequency if unset

Fall back to VBT based backlight modulation frequency if it's not
set. Do not hard code.

This could be a problem if there is no VBT.

Cc: Clint Taylor <clinton.a.taylor@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Tested-by: Clint Taylor <Clinton.A.Taylor@intel.com>
Reviewed-by: Clint Taylor <Clinton.A.Taylor@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>