Still digging up the actual VBT info for this, but wanted to get this
out there for testing, or in case others are also bugged by this.

This can happen if you boot with an external display connected.  In that
case, the attached eDP backlight modulation frequency may not be
programmed, so we need to use something (in this case the value my BIOS
normally programs with just the internal display enabled).

v2: fix masking and magic value in read_blc_pwm_ctl (Jani)
Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=67732
Tested-by: shui yangwei <yangweix.shui@intel.com> (v1)
Reviewed-by: Jani Nikula <jani.nikula@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>