drm/i915/selftests: Fixup sparse __user annotation on local var

The local var does not need the __user as it exists on the kernel stack
and not a pointer into the __user address space.

drivers/gpu/drm/i915/gem/selftests/i915_gem_mman.c:989:9: warning: dereference of noderef expression
drivers/gpu/drm/i915/gem/selftests/i915_gem_mman.c:990:13: warning: dereference of noderef expression

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200106114234.2529613-1-chris@chris-wilson.co.uk