fbdev: matroxfb: Fix compilation after fb_var_screeninfo change

Commit fb21c2f4 ("fbdev: Add FOURCC-based format configuration API")
modified the layout of the fb_var_screeninfo structure. Remove zero
fields from the static initializers in the matroxfb driver accordingly.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Florian Tobias Schandinat <FlorianSchandinat@gmx.de>