omapfb: Condition mutex acquisition

This fixes a bug introduced by this commit ID:

  commit 537a1bf0


  Author: Krzysztof Helt <krzysztof.h1@wp.pl>
  Date:   Tue Jun 30 11:41:29 2009 -0700

    fbdev: add mutex for fb_mmap locking

In which a mutex was added when changing smem_start and smem_len fields,
so the mutex inside the fb_mmap() call is actually used.

The problem was that set_fb_fix, which modifies the above 2 fields,
was called before and after registering the framebuffer,
which when used before registration, lead to a failed attempt to
use an uninitialized mutex.

Solution: Don't use mutex before framebuffer registration.

Signed-off-by: Sergio Aguirre <saaguirre@ti.com>
Acked-by: Tomi Valkeinen <tomi.valkeinen@nokia.com>
Acked-by: Imre Deak <imre.deak@nokia.com>
Tested-by: Cory Maccarrone <darkstar6262@gmail.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>