drm/syncobj: Add a syncobj_array_find helper

The wait ioctl has a bunch of code to read an syncobj handle array from
userspace and turn it into an array of syncobj pointers.  We're about to
add two new IOCTLs which will need to work with arrays of syncobj
handles so let's make some helpers.

Signed-off-by: Jason Ekstrand <jason@jlekstrand.net>
Signed-off-by: Dave Airlie <airlied@redhat.com>