drm/i915: Fix pipe CSC post offset calculation

drm/i915: Fix pipe CSC post offset calculation

commit 32cf0cb0294814cb1ee5d8727e9aac0e9aa80d2e upstream.

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
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>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>