fbdev: fbmem: remove positive test on unsigned values

fb_image.dx, fb_image.dy and fbconf2bmap.framebuffer are __u32

Cc: Jean-Christophe Plagniol-Villard <plagnioj@jcrosoft.com>
Cc: Tomi Valkeinen <tomi.valkeinen@ti.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Fabian Frederick <fabf@skynet.be>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>