gpu: ipu-v3: limit pixel clock divider to 8-bits

The DI pixel clock divider bit field is only 8 bits wide for the
integer part, so limit the divider to the 1...255 interval before
deciding whether the internal clock can be used and before writing
to the register.

Reported-by: Felix Mellmann <felix.mellmann@gmail.com>
Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>