cirrusfb: GD5434 (aka SD64) support fixed

Fix handling of the Cirrus Logic GD5434 chip.  Distinguish this chip from the
GD5430.  It allows detecting memory size for both models correctly.

Signed-off-by: Krzysztof Helt <krzysztof.h1@wp.pl>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>