drm/vmwgfx: correct fb_fix_screeninfo.line_length

Previously, the vmwgfx_fb driver would allow users to call FBIOSET_VINFO, but it would not adjust
the FINFO properly, resulting in distorted screen rendering. The patch corrects that behaviour.

See https://bugs.gentoo.org/show_bug.cgi?id=494794

 for examples.

Cc: stable@vger.kernel.org
Signed-off-by: Christopher Friedt <chrisfriedt@gmail.com>
Reviewed-by: Thomas Hellstrom <thellstrom@vmware.com>