drm/msm/sde: add generic support for drm connectors

Add generic drm_connector support to sde driver. Place
all reusable code for common drm connector properties and
state handling inside 'sde_connector.'

Change-Id: Iff1871710e7261450d1f23fa6f0938c25b5fcb45
Signed-off-by: Clarence Ip <cip@codeaurora.org>