drm/i915/perf: simplify OA unit enabling on gen7

In commit d7965152 ("drm/i915: Enable i915 perf stream for
Haswell OA unit") the enable/disable vfunc hadn't appear yet and the
same function would deal with enabling/disabling the OA unit.

This was split later on for gen8 but the gen7 retained some code that
isn't actually useful anymore.

Signed-off-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180326090831.22686-4-lionel.g.landwerlin@intel.com