drm/i915: Expand documentation for gen12 DP pre-enable sequence

drm/i915: Expand documentation for gen12 DP pre-enable sequence

Rather than just specifying the bullet numbers from the bspec (e.g.,
"4.b") actually include the description of what the bspec wants us to
do.  Steps can be renumbered or moved so including the description will
help us match the code up to the spec.  Plus if we add support for new
platforms, some of the steps may be added/removed so more descriptive
comments will be useful for ensuring all of the bspec requirements are
met.

Cc: José Roberto de Souza <jose.souza@intel.com>
Cc: Lucas De Marchi <lucas.demarchi@intel.com>
Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191107174527.11165-1-matthew.d.roper@intel.com
Reviewed-by: Lucas De Marchi <lucas.demarchi@intel.com>
Reviewed-by: José Roberto de Souza <jose.souza@intel.com>