drm/msm: Add PLL_DELTA property to HDMI connector

Clock recovery needs to update HDMI clock in order to
compensate the clock drift, so add a connector property
named as PLL_DELTA to support it. Meanwhile add a node
in debugfs to expose this functionality.

CRs-Fixed: 2015827
Change-Id: Ifdc7134b33102f112a8e3c659fae6a017ff11461
Signed-off-by: Ray Zhang <rayz@codeaurora.org>