drm/i915/perf: add missing delay for OA muxes configuration

This was dropped from the original patch series, we weren't sure
whether it was needed at the time. More recent tests show it's
definitely needed to have acurate performance data.

Signed-off-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Fixes: 19f81df2

 ("drm/i915/perf: Add OA unit support for Gen 8+")
Acked-by: Chris Wilson <chris@chris-wilson.co.uk>
[ickle: combine duplicate code and comments]
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20190710105524.23017-1-chris@chris-wilson.co.uk