drm/i915: Fix pipe CSC post offset calculation

We were miscalculating the pipe CSC post offset for the full->limited
range conversion. The resulting post offset was double what it was
supposed to be, which caused blacks to come out grey when using
limited range output on HSW+.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=71769
Cc: stable@vger.kernel.org
Tested-by: Lauri Mylläri <lauri.myllari@gmail.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>