drm/i915/dp: Use the existing write_infoframe() for DSC PPS SDPs

Infoframes are used to send secondary data packets. This patch
adds support for DSC Picture parameter set secondary data packets
in the existing write_infoframe helpers.

v3:
* Unused variables cleanup (Ville)
v2:
* Rebase on drm-tip (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@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181128202628.20238-9-manasi.d.navare@intel.com