psr2: Create a flag for Panel Self Refresh 2/SU Feature

PSR2/PSR-SU is a feature that allows the display panel to only refresh
the regions of the screen that were actually changed or damaged.

This change adds a flag so the feature can be easily enabled or
disabled.

BUG=b:232858482
TEST=build and deploy

Change-Id: I2aa24065989afbfce1996d852c42a01c97b731aa
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4692402
Reviewed-by: Scott Violet <sky@chromium.org>
Reviewed-by: Mitsuru Oshima <oshima@chromium.org>
Commit-Queue: Femi Adegunloye <mrfemi@google.com>
Cr-Commit-Position: refs/heads/main@{#1173791}