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

include/drm/drm_kunit_helpers.h

@@ -3,6 +3,8 @@
 #ifndef DRM_KUNIT_HELPERS_H_
 #define DRM_KUNIT_HELPERS_H_
 
+#include <drm/drm_drv.h>
+
 #include <linux/device.h>
 
 #include <kunit/test.h>