drm/i915/perf: Drop redundant check for perf.initialised on reset

As we cannot have an exclusive stream set if the perf has not been
initialized, we only need to check for that exclusive stream.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Cc: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20170810175743.25401-3-chris@chris-wilson.co.uk


Reviewed-by: Matthew Auld <matthew.auld@intel.com>