drm/msm/dsi-staging: make core supplies optional

Make core supplies optional for controller starting from
SDM845 since core supplies will be now voted using common
voting framework.

Change-Id: I70d05b045ac9fdc6fd3269d5ef343824f8ef6ae4
Signed-off-by: Shashank Babu Chinta Venkata <sbchin@codeaurora.org>