drm/dp_helper: Add Helpers for FRL Link Training support for DP-HDMI2.1 PCON

This patch adds support for configuring a PCON device,
connected as a DP branched device to enable FRL Link training
with a HDMI2.1 + sink.

v2: Fixed typos and addressed other review comments from Uma Shankar.
-changed the commit message for better clarity (Uma Shankar)
-removed unnecessary argument supplied to a drm helper function.
-fixed return value for max frl read from pcon.

v3: Removed DPCD 0x3035 for MAX Sink FRL b/w as per new version of spec.

Signed-off-by: Ankit Nautiyal <ankit.k.nautiyal@intel.com>
Reviewed-by: Uma Shankar <uma.shankar@intel.com> (v2)
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20201218103723.30844-5-ankit.k.nautiyal@intel.com