drm: lcdif: change burst size to 256B

If a axi bus master with a higher priority do a lot of memory access
FIFO underruns can be inspected. Increase the burst size to 256B to
avoid such underruns and to improve the memory access efficiency.

Fixes: 9db35bb3 ("drm: lcdif: Add support for i.MX8MP LCDIF variant")
Signed-off-by: Marco Felsch <m.felsch@pengutronix.de>
Reviewed-by: Marek Vasut <marex@denx.de>
Signed-off-by: Marek Vasut <marex@denx.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20221101164615.778299-1-m.felsch@pengutronix.de