msm: mdss: allow not having frame buffer memory

If pan_display is not used, the frame buffer memory is not reserved
and allocated. Adding checks to make sure no memory operations on
framebuffer.

Change-Id: Ia837d4ea0cad4e40e52ee5817c8d58e7db852a97
Signed-off-by: Huaibin Yang <huaibiny@codeaurora.org>