fbdev: xilinxfb: Use FB blanking constants

Fbdev and its drivers use FB_BLANK constants for controlling the
display's blanking state. Semantics differ to similar VESA blanking
constants.

Replace the VESA constant in xilinxfb with FB_BLANK.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Signed-off-by: Helge Deller <deller@gmx.de>