fbdev: fbmem: behave better with small rotated displays and many CPUs

Blitting an image with "negative" offsets is not working since there
is no clipping. It hopefully just crashes. For the bootup logo, there
is protection so that blitting does not happen as the image is drawn
further and further to the right (ROTATE_UR) or further and further
down (ROTATE_CW). There is however no protection when drawing in the
opposite directions (ROTATE_UD and ROTATE_CCW).

Add back this protection.

The regression is 20-odd years old but the mindless warning-killing
mentality displayed in commit 34bdb666 ("fbdev: fbmem: remove
positive test on unsigned values") is also to blame, methinks.

Fixes: 448d4797 ("fbdev: fb_do_show_logo() updates")
Signed-off-by: Peter Rosin <peda@axentia.se>
Cc: Tomi Valkeinen <tomi.valkeinen@ti.com>
Cc: Fabian Frederick <ffrederick@users.sourceforge.net>
Cc: Geert Uytterhoeven <geert+renesas@glider.be>
cc: Geoff Levand <geoff@infradead.org>
Cc: James Simmons <jsimmons@users.sf.net>
Signed-off-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>