drm/arm/malidp: Fix possible dereference of NULL

There is possible deference of NULL pointer on return of
malidp_duplicate_plane_state() if kmalloc fails. Check the
returned kmalloc pointer before continuing.

Signed-off-by: Shailendra Verma <Shailendra.v@samsung.com>
[cleaned up the code and re-formatted the commit message]
Signed-off-by: Liviu Dudau <Liviu.Dudau@arm.com>