drm/i915/dp: Configure Display stream splitter registers during DSC enable

Display Stream Splitter registers need to be programmed to enable
the joiner if two DSC engines are used and also to enable
the left and the right DSC engines. This happens as part of
the DSC enabling routine in the source in atomic commit.

v4:
* Remove redundant comment (Ville)
v3:
* Use cpu_transcoder instead of encoder->type (Ville)
v2:
* Rebase (Manasi)

Cc: Jani Nikula <jani.nikula@linux.intel.com>
Cc: Ville Syrjala <ville.syrjala@linux.intel.com>
Cc: Anusha Srivatsa <anusha.srivatsa@intel.com>
Signed-off-by: Manasi Navare <manasi.d.navare@intel.com>
Reviewed-by: Anusha Srivatsa <anusha.srivatsa@intelcom>
Link: https://patchwork.freedesktop.org/patch/msgid/20181128202628.20238-11-manasi.d.navare@intel.com