drm/i915/perf: drop pointless static qualifier in i915_perf_add_config_ioctl()

There is no need to have the 'T *v' variable static
since new value always be assigned before use it.

Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Mao Wenan <maowenan@huawei.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20191204010154.152396-1-maowenan@huawei.com