imx-drm: Move IPU_PIX_FMT_GBR24 definition into imx-ipu-v3.h

The IPU display controller supports a non-standard green-red-blue ordered
format that is used on the connection between IPU display interface 1 and
the TV encoder on i.MX53.
In preparation for the move of IPU core code out of staging, place the
IPU_PIX_FMT_GBR24 definition in imx-ipu-v3.h, so that both the IPU
display interface driver and the TVE encoder driver can access it.

Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>