drm/vc4: tests: Remove vc4_dummy_plane structure

drm/vc4: tests: Remove vc4_dummy_plane structure

The vc4_dummy_plane structure was introduced as a mean to add
mock-specific fields.

However, we never really used it and it's still strictly equivalent to
vc4_plane (which is in the same situation vs drm_plane), so we can
simply remove the vc4_dummy_plane structure and make the mock code
cleaner.

Reviewed-by: Maíra Canal <mcanal@igalia.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240527-kms-hdmi-connector-state-v15-26-c5af16c3aae2@kernel.org
Signed-off-by: Maxime Ripard <mripard@kernel.org>