drm/dp_helper: Add support for link failure detection

There are specific DPCDs defined for detecting link failures between
the PCON and HDMI sink and check the link status. In case of link
failure, PCON will communicate the same using an IRQ_HPD to source.
HDMI sink would have indicated the same to PCON using SCDC interrupt
mechanism. While source can always read final HDMI sink's status using
I2C over AUX, it is easier and faster to read the PCONs already read
HDMI sink status registers.

This patch adds the DPCDs required for link failure detection and
provide a helper function for printing error count/lane which might
help in debugging the link failure issues.

v2: Addressed comments from Uma Shankar:
-rephrased the commit message, as per the code.
-fixed styling issues
-added documentation for the helper function.

Signed-off-by: Swati Sharma <swati2.sharma@intel.com>
Signed-off-by: Ankit Nautiyal <ankit.k.nautiyal@intel.com>
Reviewed-by: Uma Shankar <uma.shankar@intel.com>
[Jani: Fixed checkpatch PARENTHESIS_ALIGNMENT.]
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20201218103723.30844-6-ankit.k.nautiyal@intel.com