drm/bridge: add support to specify preferred mode

Add the support of setting preferred display mode in lt9611
driver, so users can select which display mode to use.

CRs-Fixed: 2186423
Change-Id: I2ad4a0e99f13783f954c4177536d6ce48f5f65fd
Signed-off-by: Ray Zhang <rayz@codeaurora.org>