drm: move drm_lastclose() to drm_fops.c

Try to keep all functions that handle DRM file_operations in drm_fops.c
so internal helpers can be marked static later.

This makes the split between the 3 core files more obvious:
 - drm_stub.c: DRM device allocation/destruction and management
 - drm_fops.c: DRM file_operations (except for ioctl)
 - drm_drv.c: Global DRM init + ioctl handling
Well, ioctl handling is still spread throughout hundreds of source files,
but at least the others are clearly defined this way.

Signed-off-by: David Herrmann <dh.herrmann@gmail.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>