msm: mdss: use external display as wrapper for operations

Use the external display as a wrapper for operations making
it necessary for clients to only need a pointer to the external
display platform device. The external display will implement the
book keeping required to map operations to the correct display
interface.

CRs-Fixed: 1009284
Change-Id: I1f817e0c720dda0a9b1778f6aad653218ff9be60
Signed-off-by: Tatenda Chipeperekwa <tatendac@codeaurora.org>