imx-drm: remove imx-fb.c

imx-fb.c doesn't need to be separate from imx-drm-core.c - all it is
doing is setting up the minimum and maximum sizes of the scanout
buffers, and setting up the mode_config function pointers.  Move the
contents into imx-drm-core.c and kill this file.

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>