drm/i915/dp: Expose force_dsc_enable through debugfs

Currently we use force_dsc_enable to force DSC from IGT, but
we dont expose this value to userspace through debugfs.
This patch exposes this through the same dsc_fec_support
debugfs node per connector so that we can restore its value
back after the tests are completed.

Suggested-by: Imre Deak <imre.deak@intel.com>
Cc: Imre Deak <imre.deak@intel.com>
Cc: Lyude Paul <lyude@redhat.com>
Cc: Anusha Srivatsa <anusha.srivatsa@intel.com>
Signed-off-by: Manasi Navare <manasi.d.navare@intel.com>
Reviewed-by: Imre Deak <imre.deak@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190405224821.32435-1-manasi.d.navare@intel.com