imx-drm: simplify setup of panel format

The encoder format passed into imx_drm_crtc_panel_format*() is the
encoder format used for DRM in most cases; the HDMI encoder sets
this to none, but this is incorrect, it should be TMDS.

Since this is the case, we can pass the drm_encoder structure
directly into this function and use the supplied fields there to
configure the CRTC.

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>