tests/pm_rps: quiescent harder

tests/pm_rps: quiescent harder

Apparently some rps chips are prone to stick to max once you're there.
And all the synchronous waits in gem_quiescent_gpu are _really_ good
at forcing the kernel to go full throttle.

And even a positively tiny load (one MI_STORE_IMM per 50 ms) is enough
to eternally keeep it there, which means the first idle measurement of
the freqs before we do the reset it completely busted.

Just add a wait before the tricky test and call it a day.

Option b) would have been to now have this tiny load, but I think a
good reminder how quirky rps is is always welcome. Just in case we
start sleeping too easy at night.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=74956
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>