drm/i915/dp: Modify VDSC helpers to configure DSC for Bigjoiner slave

Make vdsc work when no output is enabled. The big joiner needs VDSC
on the slave, so enable it and set the appropriate bits.
So remove encoder usage from dsc functions.

Signed-off-by: Manasi Navare <manasi.d.navare@intel.com>
Reviewed-by: Animesh Manna <animesh.manna@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20201117194718.11462-5-manasi.d.navare@intel.com