The current code tracks business across all pipes, but we're only
really interested in the one pipe DRRS is enabled on. Fairly tiny
optimization, but something I noticed while reading the code. But it
might matter a bit when e.g. showing a video or something only on the
external screen, while the panel is kept static.

Also regroup the code slightly: First compute new bitmasks, then take
appropriate actions.

Cc: Ramalingam C <ramalingam.c@intel.com>
Cc: Sivakumar Thulasimani <sivakumar.thulasimani@intel.com>
Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>