drm/vc4: plane: Remove WARN on state being set in plane_reset

It is permitted on situations such as system resume for plane->state
to be non-NULL, and that should be handled by freeing it. Do so.

Signed-off-by: Dave Stevenson <dave.stevenson@raspberrypi.com>
Signed-off-by: Stefan Wahren <wahrenst@gmx.net>
Reviewed-by: Maxime Ripard <mripard@kernel.org>
Signed-off-by: Maíra Canal <mcanal@igalia.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20241202120343.33726-1-wahrenst@gmx.net