drm/nouveau/pwr/memx: Return debugging information

Time measured from disabling FB to re-enabling, PPWR_IN reveals status of
heads at the end of script. Helps debug various issues (like flicker).

Signed-off-by: Roy Spliet <rspliet@eclipso.eu>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>