drm/tests: Fix kunit_release_action ctx argument

The kunit_action_platform_driver_unregister is added with
&fake_platform_driver as ctx, but the kunit_release_action is called
pdev as ctx. Fix that by replacing it with &fake_platform_driver.

Fixes: 4f2b0b58 ("drm/tests: helpers: Switch to kunit actions")
Signed-off-by: Arthur Grillo <arthurgrillo@riseup.net>
Reviewed-by: Maíra Canal <mairacanal@riseup.net>
Acked-by: Maxime Ripard <mripard@kernel.org>
Signed-off-by: Maíra Canal <mairacanal@riseup.net>
Link: https://patchwork.freedesktop.org/patch/msgid/20230920-kunit-kasan-fixes-v1-1-1a0fc261832d@riseup.net