drm/i915: Warn if trying to poke a VLV backlight on invalid pipe

VLV/CHV have backlight controls only on pipes A and B. Bail out
without touching registers that don't exist, and print a warning.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>