drm/tests: helpers: Add missing export

drm_kunit_device_init() is a public function meant to be used by other
tests, but isn't exported. This leads to modpost errors when the other
tests are compiled as module.

Reported-by: kernel test robot <lkp@intel.com>
Reviewed-by: Maíra Canal <mairacanal@riseup.net>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Link: https://patchwork.freedesktop.org/patch/msgid/20221128081938.742410-3-maxime@cerno.tech