drm/msm: add PLL_ENABLE property to support clock recovery

PLL_ENABLE property is used to enable or disable the PLL
update function. With this property PLL update function
only works when PLL_ENABLE is set, and all changes done
to hardware will be discarded once PLL_ENABLE is cleared.

CRs-Fixed: 2042852
Change-Id: Ia321918382b8622101cff566049284810833f63e
Signed-off-by: Ray Zhang <rayz@codeaurora.org>