mock a drm_plane in igt_check_plane_state to make the test more robust

igt_check_plane_state test crashes in drm_atomic_helper_check_plane_state
when trying to de-reference drm_plane_state->plane->dev
due to the lack of a struct drm_plane in the mock struct drm_plane_state.
Since drm_plane_state always should contain a plane, the mock also
needs a plane to be the test more robust and realistic. Add it.

Signed-off-by: Claudio Suarez <cssk@net-c.es>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/YaiWvQESctDMipjh@gineta.localdomain