drivers: mipi_dsi: dsi_mcux_2l set SMARDMA slot from dev pixfmt

Previous version hardcoded the SMARTDMA slot to either
RGB565 or RGB565_SWAP, but that would be incorrect
if the pixfmt was RGB888. Use the mipi device
pixfmt to set the slot.

Signed-off-by: Mike J. Chen <mjchen@google.com>