msm: mdss: dsi: check ULPS state only for active lanes

It is possible that for certain panels, not all four data lanes are
active. After configuring the lanes to Ultra Low Power State (ULPS),
only check the status of the active lanes since the lanes that are not
used will never enter ULPS.

Change-Id: I0da358f2bd24d4472cfccb6cca9ca8b26ffa8226
Signed-off-by: Aravind Venkateswaran <aravindh@codeaurora.org>