video: fbdev: via: check the return value of kstrdup()

kstrdup() is a memory allocation function which can return NULL when
some internal memory errors happen. It is better to check the return
value of it to catch the error in time during the setup of viafb.

Signed-off-by: Xiaoke Wang <xkernel.wang@foxmail.com>
Signed-off-by: Helge Deller <deller@gmx.de>