drm/i915: fix hsw_write_dcomp() error message

That function can be used to write anything on D_COMP, not just
disable it, so print a more appropriate message.

Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Reviewed-by: Damien Lespiau <damien.lespiau@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>