backend-drm: Clean-up minor leak when destroying the DRM-backend

Fixes a minor leak due to launcher-libseatd:

Direct leak of 32 byte(s) in 1 object(s) allocated from:
    #0 0x7f15664e5037 in calloc (/usr/lib/x86_64-linux-gnu/libasan.so.6+0xaa037)
    #1 0x7f156305c59f in zalloc ../include/libweston/zalloc.h:38
    #2 0x7f156305c99b in seat_open_device ../libweston/launcher-libseat.c:114
    #3 0x7f1563056341 in weston_launcher_open ../libweston/launcher-util.c:79
    #4 0x7f156302f1e2 in drm_device_is_kms ../libweston/backend-drm/drm.c:2616
    #5 0x7f156302f751 in find_primary_gpu ../libweston/backend-drm/drm.c:2715
    #6 0x7f15630309a5 in drm_backend_create ../libweston/backend-drm/drm.c:2970
    #7 0x7f15630317ab in weston_backend_init ../libweston/backend-drm/drm.c:3162
    #8 0x7f1566025b61 in weston_compositor_load_backend ../libweston/compositor.c:8201
    #9 0x7f156640cb9e in load_drm_backend ../compositor/main.c:2596
    #10 0x7f156641193c in load_backend ../compositor/main.c:3079
    #11 0x7f1566413cc3 in wet_main ../compositor/main.c:3356
    #12 0x562ba484b179 in main ../compositor/executable.c:33
    #13 0x7f156624fcc9 in __libc_start_main ../csu/libc-start.c:308

But also use the launcher interface to actually close the DRM fd, in
mirror to what weston_launcher_open() does.

Signed-off-by: Marius Vlad <marius.vlad@collabora.com>