drm/i915: Split PPS write from DSC enable

The PPS SDP is fed into the transcoder whereas the DSC
block is (or at least can be) per pipe. Let's split these
into two distinct operations in an effort to untagle the
bigjoiner mess where we have two pipes feeding a single
transcoder.

Reviewed-by: Manasi Navare <manasi.d.navare@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20211022103304.24164-7-ville.syrjala@linux.intel.com