riva/fbdev: fix several -Wuninitialized

Fix several -Wuninitialized compiler warnings by changing the
order of getting modedb in riva_update_default_var() to set
first the fallback and then the prefered timing.

Signed-off-by: Danny Kukawka <danny.kukawka@bisect.de>
Signed-off-by: Florian Tobias Schandinat <FlorianSchandinat@gmx.de>