msm: mdss: dsi: ensure lanes are idle prior to ULPS entry

It is possible that the ULPS entry request can potentially overlap with
the last DCS command that was sent out on the link. This is more likely
if the DCS command was sent in LP mode. In such cases, the panel could
see some issues in the LP transmission which may trigger an error when a
BTA is done subsequently. To address this, ensure that the lanes are
idle by checking that all active FIFOs are empty and the all active data
lanes are in stop state.

CRs-Fixed: 961817
Change-Id: Ia2575f06762f6bc847f64fb70e96a275712d9135
Signed-off-by: Aravind Venkateswaran <aravindh@codeaurora.org>