msm: mdss: Hold MDP clock Ref count in pan_display path

If handoff fails, the clock ref count becomes 0 and iommu
will get detached. In pan display path, since at that
particular instant iommu gets detached, we use physical
address. After this if iommu is again attached and physical
address is fetched, it can result in continuous IOMMU page
faults. To address this hold MDP clock ref count
in pan display path.

Change-Id: I2370b276acdced2cf22083eb80fcad1b2e454315
Signed-off-by: Jayant Shekhar <jshekhar@codeaurora.org>