viafb: Retain GEMODE reserved bits

Commit c3e25673 (viafb: 2D engine rewrite)
changed the setting of the GEMODE register so that the reserved bits are no
longer preserved.  Fix that; at the same time, move this code to its own
function and restore the use of symbolic constants.

Cc: Florian Tobias Schandinat <FlorianSchandinat@gmx.de>
Cc: ScottFang@viatech.com.cn
Cc: JosephChan@via.com.tw
Signed-off-by: Jonathan Corbet <corbet@lwn.net>