fb: Added delay after changing LCD controller clock

Without the delay, the system could hang at boot. However, whether it
would hang or not depended on how the code aligned to cache line
boundaries (32 bytes): of the 8 possible alignments there were 2 where
there was no cache line boundary crossed between the clock enable and
controller enable and those two hung; the other 6 alignments booted
just fine because a cache miss caused a delay.