viafb: Retain GEMODE reserved bits
viafb: Retain GEMODE reserved bits

Commit c3e25673843153ea75fda79a47cf12f10a25ca37 (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 <9843642cd7809d7c6d8c25ac9e4b0e2f1b5283bf@gmx.de>
Cc: e8ec90cdf6f7bc58dc5d9632b91cc6e0ba5c4c09@viatech.com.cn
Cc: 19be70ac8f4a77f60db36db8c0e304c340d00e2f@via.com.tw
Signed-off-by: Jonathan Corbet <b2ce64d5587c02f12e367eac59751145a0660c51@lwn.net>

KristFoundation/Programs,KristFoundation/Programs,TeamVee-Kanas/android_kernel_samsung_kanas,KristFoundation/Programs,KristFoundation/Programs,TeamVee-Kanas/android_kernel_samsung_kanas,TeamVee-Kanas/android_kernel_samsung_kanas,TeamVee-Kanas/android_kernel_samsung_kanas,KristFoundation/Programs,TeamVee-Kanas/android_kernel_samsung_kanas,KristFoundation/Programs