drm/i915: Fix blank panel at reopening lid

When you reopen the lid on a laptop with PCH, the panel suddenly goes
blank sometimes.  It seems because BLC_PWM_CPU_CTL register is cleared
to zero when BLC_PWM_CPU_CTL2 and BLC_PWM_PCH_CTL1 registers are
enabled.

This patch fixes the problem by moving the call of the function setting
BLC_PWM_CPU_CTL after enabling other two registers.

Reported-and-tested-by: Hugh Dickins <hughd@google.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>