updatescrollmode() must not select ywrap scrolling if
divides(vc->vc_font.height, yres) is not true as this is not supported by
the actual ywrap scrolling code.

The bug is triggered with e.g.  mode 800x600, vxres 1024, vyres 8192, bpp
8, font dimensions 8x16, 8Mb video ram and FBINFO_HWACCEL_YWRAP set.  If
those conditions are met, scrolling is broken and garbage is permanently
displayed at the bottom of the screen.

No regression, no possible side effects.

Definitely needed by cyblafb and probably needed by amifb.
Signed-off-by: NKnut Petersen <Knut_Petersen@t-online.de>
Acked-by: NAntonino Daplas <adaplas@pol.net>
Signed-off-by: NAndrew Morton <akpm@osdl.org>
Signed-off-by: NLinus Torvalds <torvalds@osdl.org>