drm: fix fbs in DRM_IOCTL_MODE_GETRESOURCES ioctl

The DRM_IOCTL_MODE_GETRESOURCES ioctl just returns bogus framebuffers.
That is because the framebuffers for each file are in the filp_head
member of struct drm_framebuffer, not in the head member.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: Dave Airlie <airlied@redhat.com>