drm/tests: helpers: Include missing drm_drv header

We have a few functions declared in our kunit helpers header, some of
them dereferencing the struct drm_driver.

However, we don't include the drm_drv.h header file defining that
structure, leading to compilation errors if we don't include both
headers.

Fixes: d9878031 ("drm/tests: helpers: Allow to pass a custom drm_driver")
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-v7-1-8f4af575fce2@kernel.org