drm/tests: helpers: Add atomic helpers

The mock device we were creating was missing any of the driver-wide
helpers. That was fine before since we weren't testing the atomic state
path, but we're going to start, so let's use the default
implementations.

Reviewed-by: Maíra Canal <mcanal@igalia.com>
Signed-off-by: Maxime Ripard <mripard@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20240222-kms-hdmi-connector-state-v7-2-8f4af575fce2@kernel.org