fbcon: Use array3_size() helper in scr_memcpyw()

Use array3_size() helper instead of the open-coded version in scr_memcpyw()
and scr_memsetw(). These sorts of multiplication factors need to be wrapped
in array3_size().

This issue was found with the help of Coccinelle and, audited and fixed
manually.

Addresses-KSPP-ID: https://github.com/KSPP/linux/issues/83


Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Signed-off-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200615231542.GA20470@embeddedor