msm: mdss: Change return value in dsi_off

Match panel_power_off check return value, during
dsi off, with panel_power_on, during dsi on. Otherwise,
this leads to inconsistency, when performing random
dsi on and off sequence.

CRs-Fixed: 608203
Change-Id: I0ed7c8d6ebe846492480e36fe8958909927b82b9
Signed-off-by: Mayank Chopra <makchopra@codeaurora.org>