drm/msm/sde: add connector api for querying display info

Provide an API to allow querying of the physical display info
for each sde connector so that other parts of the driver don't
have to access the underlying physical display drivers directly.

Change-Id: Ic3534e99ef414c12ee6a9cdd2ce97745a3aa9001
Signed-off-by: Clarence Ip <cip@codeaurora.org>