[CG6]: accelerated copyarea

This patch adds accelerated copyarea and sets READS_FAST flag.
This doubles scrolling speed on SparcStation20 85MHz.

It also fixes a comment in cg6_fillrect function.

Signed-off-by: Krzysztof Helt <krzysztof.h1@wp.pl>
Signed-off-by: David S. Miller <davem@davemloft.net>