msm: mdss: fix random panel power off failure in LP2 state

Return value is not initialize in mdss_dsi_panel_power_ctrl resulting in
failure of power off calls. When LP2 power state is requested and
DSI core is in power on state because uninitialized ret is returned.

Fix issue by initialize return value.

CRs-Fixed: 1055265
Change-Id: I5e9f175f4f0c97a1913a3577138ecc43b6862bc9
Signed-off-by: Sachin Bhayare <sachin.bhayare@codeaurora.org>