dsp_fbexport: use kzalloc

The dsp_export function calls omapfb_register_client() right after
allocating the memory with kmalloc.
A commit added to omapfb_register_client() a check on the value of
plane_idx in the notified block structure. As we're only using kmalloc,
you may end-up comparing garbage/random values (It even leads to a oops
on my n770).

Using kzalloc() fixes the issue.

Signed-off-by: Arnaud Patard <arnaud.patard@rtp-net.org>
Signed-off-by: Tony Lindgren <tony@atomide.com>