platform/msm: use extcon API in msm-ext-disp module

Recent external connector changes moved the 'struct extcon_dev'
of extcon subsystem to driver/extcon/extcon.h not allowing users
to access extcon_dev fields.
Patch replaces direct extcon_dev fields access with extcon API calls.

Change-Id: I6273d9d142b8eebaa367041b38429209899229e2
Signed-off-by: Alexander Beykun <abeykun@codeaurora.org>