video: fbdev: pvr2fb: remove unnecessary comparison of unsigned integer with < 0

There is no need to compare *var->xoffset* or *var->yoffset* with < 0
because such variables are of type unsigned, making it impossible to
hold a negative value.

Fix this by removing such comparisons.

Addresses-Coverity-ID: 1451964 ("Unsigned compared against 0")
Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Signed-off-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190722203358.GA29111@embeddedor