drm/i915/cnp: Don't touch other PCH clock gating bits.

Don't touch other bits. My bad.

I haven't seen any case where those other bits appeard to be
set before we touch it, but it is safe to avoid touching
other bits we weren't told to touch.

Fixes: 0a46ddd5

 ("drm/i915/cnp: Wa 1181: Fix Backlight issue")
Cc: Dhinakaran Pandiyan <dhinakaran.pandiyan@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Reviewed-by: Dhinakaran Pandiyan <dhinakaran.pandiyan@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20170908234534.17986-1-rodrigo.vivi@intel.com