Whenever ywrap scrolling is selected together with 180 degree screen
rotation, 2.6.15-rc6 and earlier versions are broken. fb_pan_display()
expects non-negative yoffsets, but ud_update_start() calls it with
yoffsets down to -(yres - font height). This patch transforms yoffset
to the correct range 0 ... vyres-1.
Some obviously unneeded parentheses are removed, too.
Verified with cyblafb, should be applied before 2.6.15-final because it
does fix the framebuffer rotation code introduced early in the 2.6.15
release cycle.