drm/msm/dp: disable mainlink clocks when controller is turned off

Disable the mainlink clocks when the controller is turned off
via the HPD-off path. This will ensure that the mainlink clocks
are disabled only if the HPD-on path has been successfully
triggered. Attempting to disable the mainlink clocks otherwise
will result in a failure since the clock unprepare will be
called without corresponding clock prepare.

CRs-Fixed: 2101790
Change-Id: Iedb12f9ca3f2842ecf1992ad1c1076b839fd4eda
Signed-off-by: Tatenda Chipeperekwa <tatendac@codeaurora.org>