omapfb: Reorder Register_framebuffer call

This fixes the issue in which mm_lock mutex was attempted to be
used without initializing previously.

Thanks to the testers!
 - OMAP3430 SDP (Anand Gadiyar)
 - OMAP3530 EVM (Vaibhav Hiremath)
 - LogicPD's OMAP boards (Peter Brada)
 - Beagleboard Rev. C2 (Eric Witcher)

Signed-off-by: Sergio Aguirre <saaguirre@ti.com>
Tested-by: Vaibhav Hiremath <hvaibhav@ti.com>
Tested-by: Anand Gadiyar <gadiyar@ti.com>
Tested-by: Peter Barada <peterb@logicpd.com>
Tested-by: Eric Witcher <ewitcher@mindspring.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>