drm/i915/perf: remove redundant power configuration register override

The caller of i915_oa_init_reg_state() already sets this.

Signed-off-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200317132222.2638719-2-lionel.g.landwerlin@intel.com