msm: mdss: fix parsing of external display platform device

Fix the parsing of the external display platform device by making
use of the framebuffer and display interface platform devices to
find the correct handle.

CRs-Fixed: 1077559
Change-Id: Ifde6d8e042eb651893a1760bdc942a349bf190b7
Signed-off-by: Tatenda Chipeperekwa <tatendac@codeaurora.org>