drm/i915/drrs: Restrict buffer tracking to the DRRS pipe

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>