drm/dp_helper: Add support for Configuring DSC for HDMI2.1 Pcon

This patch adds registers for getting DSC encoder capability for
a HDMI2.1 PCon. It also addes helper functions to configure
DSC between the PCON and HDMI2.1 sink.

v2: Corrected offset for DSC encoder bpc and minor changes.
Also added helper functions for getting pcon dsc encoder capabilities
as suggested by Uma Shankar.

v3: Only setting the DSC bits for the Protocol Converter control
registers, avoiding overwritining color conversion bits.

Signed-off-by: Ankit Nautiyal <ankit.k.nautiyal@intel.com>
Reviewed-by: Uma Shankar <uma.shankar@intel.com> (v2)
[Jani: Fixed checkpatch BLOCK_COMMENT_STYLE.]
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20201218103723.30844-7-ankit.k.nautiyal@intel.com