drm/i915/dc3co: Add description of how it works

Add a basic description about how DC3CO works to help people not
familiar with it.

While at it, I also improved the delayed work handle and function
names and removed a debug message that is ambiguous and not much
useful, no changes in behavior here.

Cc: Anshuman Gupta <anshuman.gupta@intel.com>
Cc: Imre Deak <imre.deak@intel.com>
Signed-off-by: José Roberto de Souza <jose.souza@intel.com>
Reviewed-by: Imre Deak <imre.deak@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200205214945.131012-1-jose.souza@intel.com