improved reads of CRT controller and timing PROM

The compucolor had a place for a small PROM holding timing parameters
which get written to the smc5027 CRT controller, but in fact the PROM
wasn't populated and the compucolor used a masked ROM version of the
5027 chip.  Also, the 5027 chip can only be written to, not read, on the
compucolor.  The new code models this behavior.