drm/i915/perf: remove redundant oa buffer initialization

We initialize the OA buffer everytime we enable the OA unit (first call in
gen[78]_oa_enable), so we don't need to initialize when preparing the metric
set.

Signed-off-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Reviewed-by: Lucas De Marchi <lucas.demarchi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181023100707.31738-3-lionel.g.landwerlin@intel.com