drivers: i915: Fix BLC PWM register setup

There is an error in i915_read_blc_pwm_ctl, where the register values
are not being copied correctly.  BLC_PWM_CTL and BLC_PWM_CTL2 are
getting mixed up.  This patch fixes that so that saveBLC_PWM_CTL2 and
not saveBLC_PWM_CTL is copied to the BLC_PWM_CTL2 register.

Signed-off-by: Simon Que <sque@chromium.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>