disp: msm: dp: report HDR10+ parameters with other HDR properties

Report the HDR10+ sink capability and payload data when user-space
reads the "hdr" debugfs node. Also add support for reporting HDR
properties for MST sessions via the new "hdr_mst" debugfs node.
Write support for this node was removed since it updated the
connector state in an inconsistent way, therefore HDR updates must
come from the atomic commit.

Change-Id: I58af4042c1b3198eb78fe413728104071cf50caf
Signed-off-by: Steve Cohen <cohens@codeaurora.org>