viafb: use proper register for colour when doing fill ops

The colour was written to a wrong register for fillrect operations.
This sometimes caused empty console space (for example after 'clear')
to have a different colour than desired. Fix this by writing to the
correct register.
Many thanks to Daniel Drake and Jon Nettleton for pointing out this
issue and pointing me in the right direction for the fix.

Fixes http://dev.laptop.org/ticket/9323



Signed-off-by: Florian Tobias Schandinat <FlorianSchandinat@gmx.de>
Cc: Joseph Chan <JosephChan@via.com.tw>
Cc: Daniel Drake <dsd@laptop.org>
Cc: Jon Nettleton <jon.nettleton@gmail.com>
Cc: stable@kernel.org