staging: dgap: remove unnecessary if statement for checking NULL

dgap_release_remap() function is only called after the memory
has been remapped so if statement for checking NULL doesn't
need.

And also release_mem_region() calls are moved after iounmap() calls.

Signed-off-by: Daeseok Youn <daeseok.youn@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>