dm: blk: Add probe in blk_first_device/blk_next_device

The description claims that the device is probed but it isn't.

Add the device_probe() call.

Also consolidate the iteration into one function.

Fixes: 8a5cbc06 ("dm: blk: Use uclass_find_first/next_device() in blk_first/next_device()")
Signed-off-by: Michal Suchanek <msuchanek@suse.de>