video: fbdev: amifb: remove impossible condition

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

Cc: Timur Tabi <timur@tabi.org>
Cc: Tomi Valkeinen <tomi.valkeinen@ti.com>
Cc: sachiniiitm@gmail.com
Cc: ravikant.s2@samsung.com
Cc: p.shailesh@samsung.com
Cc: ashish.kalra@samsung.com
Cc: vidushi.koul@samsung.com
Signed-off-by: Sachin Shukla <sachin.s5@samsung.com>
[b.zolnierkie: split from the bigger patch]
Signed-off-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>