video: fbdev: intelfb: remove impossible condition

xoffset and yoffset of struct fb_var_screeninfo are unsigned and so
they can never be less than 0.

Signed-off-by: Sudip Mukherjee <sudip.mukherjee@codethink.co.uk>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>