sisfb: fix regression with uninitalized fb_info->mm_lock mutex

Remove redundant call to the sisfb_get_fix() before sis frambuffer is
registered.

This fixes a problem with uninitialized the fb_info->mm_lock mutex
introduced by the commit 537a1bf0

 " fbdev: add mutex for fb_mmap
locking"

Signed-off-by: Krzysztof Helt <krzysztof.h1@wp.pl>
Tested-by: Wu Zhangjin <wuzhangjin@gmail.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>