drm/i915/display/cnl+: Handle fused off DSC

DSC could be fused off, so not all GEN10+ platforms will support it.

Cc: Manasi Navare <manasi.d.navare@intel.com>
Cc: Martin Peres <martin.peres@linux.intel.com>
Reviewed-by: Ramalingam C <ramalingam.c@intel.com>
Reviewed-by: Manasi Navare <manasi.d.navare@intel.com>
Signed-off-by: José Roberto de Souza <jose.souza@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191026001323.216052-5-jose.souza@intel.com