gem_quiescent_gpu() is supposed to ensure that the HW is idle, and in
the process kick the GPU into rc6, so we should not need a long delay
afterwards to ensure that we are indeed in rc6. We do however need a
small delay in order to be sure that rc6 cycle counter has started and
stopped.

v2: Apply to rc6p as well.
v3: The longest rc6 timeout (before the HW kicks in and enables rc6 on
an idle GPU) is 50ms, so make sure that at least that time has passed
since we were busy.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>