tests/pm_pc8: try to reuse FBs when possible

tests/pm_pc8: try to reuse FBs when possible

Instead of creating a new FB every time we need one, create a cache of
FBs and reuse whenever possible. That means we'll create at most two
FBs, and reuse them hundreds and hundreds of times.

The kmstest_paint_test_pattern function takes about 1-2 seconds to
run, and we'll avoid it whenever we reuse the FB.

This makes the time taken to run the modeset-lpsp-stress subtest go
from 2:29 to 1:29.

A full "time ./pm_pc8 --quick" goes from 8:14 to 6:27.

Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>