backends/drm: Fix the closefb ioctl

The DRM_IOCTL_MODE_CLOSEFB ioctl takes a struct drm_mode_closefb as
argument and not the framebuffer id as integer.

That distinction is important because the kernel side explicitly
checks whether the padding in drm_mode_closefb has been correctly
initialized to zero:
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/drivers/gpu/drm/drm_framebuffer.c#n494

Use drm_mode_closefb and thus make sure the kernel side is not reading
garbage when testing the padding.