drm/imx: tve: fix media bus format for VGA output

Commit a7c6e76f ("drm/imx: switch to use media bus formats") accidentally
replaced IPU_PIX_FMT_GBR24 with MEDIA_BUS_FMT_YUV8_1X24 instead of the correct
MEDIA_BUS_FMT_GBR888_1X24. This patch is needed to fix VGA output in i.MX53.

Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>