drm/i915: Do not enable FEC without DSC

drm/i915: Do not enable FEC without DSC

BugLink: https://bugs.launchpad.net/bugs/1830934
commit 5aae7832d1b4ec614996ea0f4fafc4d9855ec0b0 upstream.

Currently we enable FEC even when DSC is no used. While that is
theoretically valid supposedly there isn't much of a benefit from
this. But more importantly we do not account for the FEC link
bandwidth overhead (2.4%) in the non-DSC link bandwidth computations.
So the code may think we have enough bandwidth when we in fact
do not.

Cc: stable@vger.kernel.org
Cc: Anusha Srivatsa <anusha.srivatsa@intel.com>
Cc: Manasi Navare <manasi.d.navare@intel.com>
Fixes: 240999cf339f ("i915/dp/fec: Add fec_enable to the crtc state.")
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190326144903.6617-1-ville.syrjala@linux.intel.com
Reviewed-by: Manasi Navare <manasi.d.navare@intel.com>
(cherry picked from commit 6fd3134ae3551d4802a04669c0f39f2f5c56f77d)
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Stefan Bader <stefan.bader@canonical.com>
Signed-off-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>