fb: Initialize fb_info mutexes in framebuffer_alloc()

This way they'll be properly initialized early enough for users that may
touch them before the framebuffer has been registered.

Drivers that allocate their fb_info structure some other way (like
matrocfb's broken static allocation) need to be fixed up appropriately.

Signed-off-by: Paul Mundt <lethal@linux-sh.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>