drm/i915/perf: Describe structure members in documentation

Add missing descriptions of i915_perf_stream structure members
to documentation.

Cc: Umesh Nerlige Ramappa <umesh.nerlige.ramappa@intel.com>
Cc: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Cc: Robert Bragg <robert@sixbynine.org>
Signed-off-by: Anna Karas <anna.karas@intel.com>
Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20191022101338.17048-1-anna.karas@intel.com