drm/msm/hdmi: 8996 PLL: Populate unprepare

Without doing anything in unprepare, the HDMI driver isn't able to
switch modes successfully. Calling set_rate with a new rate results
in an un-locked PLL.

If we reset the PLL in unprepare, the PLL is able to lock with the
new rate.

Signed-off-by: Archit Taneja <architt@codeaurora.org>