drm/i915: Don't unregister fbdev's fb twice

Calling drm_framebuffer_unregister_private() in intel_fbdev_destroy() is
superfluous because the framebuffer will subsequently be unregistered by
drm_framebuffer_free() when unreferenced in drm_framebuffer_remove().
The call is a leftover, when it was introduced by commit 36206361
("drm: revamp framebuffer cleanup interfaces"), struct intel_framebuffer
was still embedded in struct intel_fbdev rather than being a pointer as
it is today, and drm_framebuffer_remove() wasn't used yet.

As a bonus, the ID of the framebuffer is no longer 0 in the debug log:

Before:
    [   39.680874] [drm:drm_mode_object_unreference] OBJ ID: 0 (3)
    [   39.680878] [drm:drm_mode_object_unreference] OBJ ID: 0 (2)
    [   39.680884] [drm:drm_mode_object_unreference] OBJ ID: 0 (1)

After:
    [  102.504649] [drm:drm_mode_object_unreference] OBJ ID: 45 (3)
    [  102.504651] [drm:drm_mode_object_unreference] OBJ ID: 45 (2)
    [  102.504654] [drm:drm_mode_object_unreference] OBJ ID: 45 (1)

Signed-off-by: Lukas Wunner <lukas@wunner.de>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/5031860caad67faa0f1be5965331ef048a311a01.1465383212.git.lukas@wunner.de