disp: msm: dsi: add stub function parse power_cfg in trusted-vm

In trusted-vm, there are no power cfg entries in device-tree as
there is no support. Add stub function to avoid parsing errors
related to power cfg in trusted-vm.

Change-Id: Id28ad9a4d5608d561e22b318a08c6bc1ccc5f2ee
Signed-off-by: Veera Sundaram Sankaran <veeras@codeaurora.org>