matroxfb: fix regression with uninitalized fb_info->mm_lock mutex (second head)

Remove redundant locking by the mm_lock mutex before a second head of
matrox framebuffer is registered.

This fixes a problem with uninitialized the fb_info->mm_lock mutex
introduced by the commit 537a1bf059f " fbdev: add mutex for fb_mmap
locking"

Signed-off-by: Krzysztof Helt <krzysztof.h1@wp.pl>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>