msm: ext_display: move ext_display out of FB driver

Make ext_display a standalone module independent of
framebuffer driver, so that it could be shared by
various drivers such as framebuffer and KMS.

CRs-Fixed: 2010135
Change-Id: I336c556cbfbd66d3cb3467acaea038d5d3651f67
Signed-off-by: Ray Zhang <rayz@codeaurora.org>