drm: drop DRM_IOCTL_MODE_REPLACEFB, add+remove works just as well.

The replace fb ioctl replaces the backing buffer object for a modesetting
framebuffer object.  This can be acheived by just creating a new
framebuffer backed by the new buffer object, setting that for the crtcs
in question and then removing the old framebuffer object.

Signed-off-by: Kristian Hogsberg <krh@redhat.com>
Acked-by: Jakob Bornecrantz <jakob@vmware.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>