imx-drm: imx-drm-core: fix imx_drm_encoder_get_mux_id

The decoder mux id is equal to the port id of the encoder's input port
that is connected to the given crtc, not to the endpoint id (which is
arbitrary and usually zero).

Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>
Tested-by: Shawn Guo <shawn.guo@freescale.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>