drm/i915/perf: Initialise err to 0 before looping over ce->engines

Smatch warning that the loop may be empty causing us to check err before
it had been set. Ensure that it is initialised to 0, just in case.

v2: Refactor the inner loop for better scooping and clarity

Fixes: a9877da2 ("drm/i915/oa: Reconfigure contexts on the fly")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190726131458.8310-1-chris@chris-wilson.co.uk