drm/imx: imx-tve: use drm managed resources

Use drmm_simple_encoder_alloc() to align encoder memory lifetime with
the drm device. drm_encoder_cleanup() is called automatically before
the memory is freed.
Also fold imx_tve_register() into imx_tve_bind().

Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>