drm/vc4: tests: Use return instead of assert

The vc4_mock_atomic_add_output() and vc4_mock_atomic_del_output() assert
that the functions they are calling didn't fail. Since some of them can
return EDEADLK, we can't properly deal with it.

Since both functions are expected to return an int, and all caller check
the return value, let's just properly propagate the errors when they
occur.

Fixes: f759f5b5 ("drm/vc4: tests: Introduce a mocking infrastructure")
Fixes: 76ec18dc ("drm/vc4: tests: Add unit test suite for the PV muxing")
Reviewed-by: Maíra Canal <mcanal@igalia.com>
Link: https://lore.kernel.org/r/20250403-drm-vc4-kunit-failures-v2-1-e09195cc8840@kernel.org


Signed-off-by: Maxime Ripard <mripard@kernel.org>