drm/i915: Add support for starting FRL training for HDMI2.1 via PCON

This patch adds functions to start FRL training for an HDMI2.1 sink,
connected via a PCON as a DP branch device.
This patch also adds a new structure for storing frl training related
data, when FRL training is completed.

v2: As suggested by Uma Shankar:
-renamed couple of variables for better clarity
-tweaked the macros used for correct semantics for true/false
-fixed other styling issues.

v3: Completed the TODO for condition for going to FRL mode.
Modified the condition to determine the required FRL b/w
based only on the Pcon and Sink's max FRL values.
Moved the frl structure initialization to intel_dp_init_connector().

v4: Fixed typo in initialization of frl structure.

v5: Always use FRL if its possible, instead of enabling only for
higher modes as done in v3.

Signed-off-by: Ankit Nautiyal <ankit.k.nautiyal@intel.com>
Reviewed-by: Uma Shankar <uma.shankar@intel.com> (v2)
[Jani: Fixed checkpatch BRACES, CONSTANT_COMPARISON.]
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20201218103723.30844-10-ankit.k.nautiyal@intel.com