drm/dsi: add helpers for DSI compression mode and PPS packets

Add helper functions for sending the DSI compression mode and picture
parameter set data type packets. For the time being, limit the support
to using VESA DSC 1.1 and the default PPS. This may need updating if the
need arises for proprietary compression or non-default PPS, however keep
it simple for starters.

v2: Add missing EXPORT_SYMBOL

Cc: Vandita Kulkarni <vandita.kulkarni@intel.com>
Reviewed-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191028150047.22048-5-jani.nikula@intel.com