Build the struct drm_mm selftests so that we can trivially run them
within our CI.

"Enable debug, become developer." - Joonas Lahtinen
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170125101102.9010-3-chris@chris-wilson.co.uk