drm/i915: remove i915_perf_stream::size_exponent

i915_perf_stream::size_exponent was never used since its addition in
commit a37f08a8 (drm/i915/perf: Refactor oa object to better manage
resources). Drop it.

Found by https://github.com/jirislaby/clang-struct

.

Signed-off-by: Jiri Slaby (SUSE) <jirislaby@kernel.org>
Cc: Jani Nikula <jani.nikula@linux.intel.com>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Cc: Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>
Cc: intel-gfx@lists.freedesktop.org
Acked-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240216065326.6910-11-jirislaby@kernel.org