video: cirrusfb: Remove incorrect checks

'xoffset' and 'yoffset' are unsigned and hence cannot be less than 0.

Signed-off-by: Sachin Kamat <sachin.kamat@linaro.org>
Cc: Jeff Garzik <jgarzik@redhat.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>