drm/i915: Don't leak framebuffer_references if drm_framebuffer_init() fails

Don't increment obj->framebuffer_references until we know we actually
managed to create the framebuffer.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1452777736-4909-2-git-send-email-ville.syrjala@linux.intel.com