video: fbdev: i740fb: use memset_io() to clear screen

sparse complains that using memset() on __iomem pointer is wrong:
incorrect type in argument 1 (different address spaces)

Use memset_io() to clear screen instead.

Tested on real i740 cards.

Signed-off-by: Ondrej Zary <linux@zary.sk>
Signed-off-by: Helge Deller <deller@gmx.de>