drm/i915/dp: abstract struct intel_dp pps members to a sub-struct

Add some namespacing to highlight what belongs where. No functional
changes.

Cc: Anshuman Gupta <anshuman.gupta@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210120101834.19813-3-jani.nikula@intel.com