drm/etnaviv: populate GEM objects on cpu_prep

CPU prep is the point where we can reasonably return an error to userspace
when something goes wrong while populating the object. If we leave the
object unpopulated at this point, the allocation will happen in the
fault handler when userspace accesses the object through the mmap space,
where we don't have any other option than to OOM the system.

Signed-off-by: Lucas Stach <dev@lynxeye.de>