kunit: Setup DMA masks on the kunit device

Commit d393acce ("drm/tests: Switch to kunit devices") switched the
DRM device creation helpers from an ad-hoc implementation to the new
kunit device creation helpers introduced in commit d03c720e ("kunit:
Add APIs for managing devices").

However, while the DRM helpers were using a platform_device, the kunit
helpers are using a dedicated bus and device type.

That situation creates small differences in the initialisation, and one
of them is that the kunit devices do not have the DMA masks setup. In
turn, this means that we can't do any kind of DMA buffer allocation
anymore, which creates a regression on some (downstream for now) tests.

Let's set up a default DMA mask that should work on any platform to fix
it.

Fixes: d03c720e ("kunit: Add APIs for managing devices")
Signed-off-by: Maxime Ripard <mripard@kernel.org>
Tested-by: Guenter Roeck <linux@roeck-us.net>
Reviewed-by: David Gow <davidgow@google.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>