omapfb: Condition mutex acquisition

Acquiring mutex before framebuffer registration doesn't make sense,
As there's no danger of external access to the memory related fields.

Cc: tomi.valkeinen@nokia.com
Cc: imre.deak@nokia.com
Signed-off-by: Sergio Aguirre <saaguirre@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>