drm/i915: Add helper functions for calculating DSC parameters for HDMI2.1

The DP-HDMI2.1 PCON spec provides way for a source to set PPS
parameters: slice height, slice width and bits_per_pixel, based on
the HDMI2.1 sink capabilities. The DSC encoder of the PCON will
respect these parameters, while preparing the 128 byte PPS.

This patch adds helper functions to calculate these PPS paremeters as
per the HDMI2.1 specification.

v2: Addressed review comments given by Uma Shankar:
-added documentation for functions
-fixed typos and errors

Signed-off-by: Ankit Nautiyal <ankit.k.nautiyal@intel.com>
Reviewed-by: Uma Shankar <uma.shankar@intel.com>
[Jani: Fixed checkpatch SUSPECT_CODE_INDENT, TYPO_SPELLING]
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20201218103723.30844-14-ankit.k.nautiyal@intel.com