imx-drm: imx-drm-core: sanitise imx_drm_encoder_get_mux_id()

Address the following issues:
- imx_drm_encoder_get_mux_id() searches the CRTC list for the matching
  CRTC, and returns the position within this list as the MUX programming
  value for encoders.  This is sub-optimal for two reasons:
  1. It relies upon the CRTC list not changing during the lifetime of
     the driver.
  2. It is dependent on the initialisation order of the CRTCs.

  We address (1) in this patch, leaving (2) until a better solution can
  be found, as (2) requires larger changes.

- imx_drm_encoder is unused.  Instead, pass the drm_encoder which is
  slightly more useful; all callers pass encoder->crtc as the required
  crtc, so move this inside the function.

Acked-by: Philipp Zabel <p.zabel@pengutronix.de>
Acked-by: Shawn Guo <shawn.guo@linaro.org>
Reviewed-by: Fabio Estevam <fabio.estevam@freescale.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>