ANDROID: GKI: Add 'dsc_info' to struct drm_dp_mst_port

Resolves an abi diff. Also adds setter/getter functions which are also
exported.

Bug: 152417756
Signed-off-by: Alistair Delva <adelva@google.com>
Change-Id: Ieb567ae1915446720ed2e77651a9a49f1e295360