drm/msm/sde: add virtual writeback device and connector

Enable virtual writeback connector by allowing a property on the
connector to specify output frame buffer id, which takes in a
framebuffer object added via ADDFB2 ioctl.  A new writeback
configure ioctl is added to configure connection state and
display modes of writeback connector.

Change-Id: Ifce411a3f0798e3af7dd7f19da27d67cdd849bfb
Signed-off-by: Adrian Salido-Moreno <adrianm@codeaurora.org>
Signed-off-by: Alan Kwong <akwong@codeaurora.org>
Signed-off-by: Clarence Ip <cip@codeaurora.org>
Signed-off-by: Dhaval Patel <pdhaval@codeaurora.org>
Signed-off-by: Narendra Muppalla <narendram@codeaurora.org>