drm/i915/perf: Set the exclusive stream under perf->lock

drm/i915/perf: Set the exclusive stream under perf->lock

The BKL struct_mutex is no more, the only serialisation we required for
setting the exclusive stream is already managed by ce->pin_mutex in
gen8_configure_all_contexts(). As such, we can manipulate
i915_perf.exclusive_stream underneath our own (already held) perf->lock.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Umesh Nerlige Ramappa <umesh.nerlige.ramappa@intel.com>
Cc: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191007140812.10963-2-chris@chris-wilson.co.uk
Link: https://patchwork.freedesktop.org/patch/msgid/20191007210942.18145-2-chris@chris-wilson.co.uk