drm/i915/panel: Restore saved value of BLC_PWM_CTL

After a GPU reset, the backlight controller registers may be also reset
to 0. In that case we should restore those to the original values
programmed by the BIOS. Note that we still lack the code to handle the
case where the BIOS failed to program those registers at all...

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>