fb: JZ4770: Reorder probe function

Do not register the frame buffer before it is ready for use.

No need to explicitly set drvdata to NULL on remove, that is done for
us in recent kernels.
fb: JZ4770: Cleaned up enable/disable code

Always put a delay between enabling the clock and enabling the LCDC.
At boot this was essential to avoid hangups, so it seems unlikely it
is optional at other times.

When blanking, also shut down LCD panel.

On mode changes, wait for the frame to end before shutting down the
LCDC (normal disable instead of quick disable).