msm: mdss: dsi: fix handling of ulps_suspend feature flag

In the current implementation, when ulps_suspend feature is enabled, it
results in ULPS mode being configured even when turning off clocks
during idle screen use cases. Fix this by correcting the conditional
logic when turning off the link clocks.

CRs-Fixed: 912434
Change-Id: I44e8556afcc5110afb9e2193e8558306847e90a0
Signed-off-by: Aravind Venkateswaran <aravindh@codeaurora.org>