drm/msm/dsi: save pll state before dsi host is powered off

[ Upstream commit a1028dcfd0dd97884072288d0c8ed7f30399b528 ]

Save pll state before dsi host is powered off. Without this change
some register values gets resetted.

Signed-off-by: Harigovindan P <harigovi@codeaurora.org>
Signed-off-by: Rob Clark <robdclark@chromium.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>