drm/i915/perf: don't read head/tail pointers outside critical section

Reading or writing those fields should only happen under
stream->oa_buffer.ptr_lock.

Signed-off-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Fixes: d1df41eb ("drm/i915/perf: rework aging tail workaround")
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Acked-by: Ashutosh Dixit <ashutosh.dixit@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200330091411.37357-1-lionel.g.landwerlin@intel.com