drm/msm/sde: allow connector to accept null framebuffer id

Zero is valid a framebuffer identifier to indicated no framebuffer
attached. Change connector to accept null framebuffer as buffer
detachment.

CRs-Fixed: 2009714
Change-Id: I4ffc7e6947b31dce81631746bb2ca2daf02f978c
Signed-off-by: Alan Kwong <akwong@codeaurora.org>