Some smart panel requires a delay between command sequences, while PXA
LCD controller didn't provide such one, let's emulate this by software.

A software delay marker can be inserted into the command sequence, once
pxafb_smart_queue() detects this, it flushes the previous commands and
delay for a specified number of milliseconds.
Signed-off-by: Eric Miao <eric.miao@marvell.com>