drm/msm/dsi-staging: skip power parse when ext bridge is used

Skip power and reset sequence parse when external dsi bridge is used.

CRs-Fixed: 2351727
Change-Id: I8d5b1a352ad64773277a41ced356bbd32a267af2
Signed-off-by: Xiaowen Wu <wxiaowen@codeaurora.org>