drm/i915/lmem: Check against i915_selftest only under CONFIG_SELFTEST

The i915_selftest module parameters only exist when
CONFIG_DRM_I915_SELFTEST is set.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Matthew Auld <matthew.auld@intel.com>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191101095147.9769-1-chris@chris-wilson.co.uk