disp: msm: sde: fix panel mode check during secure transition

During transition from sui to non-secure if concurrent
writeback is enabled, it is incorrectly assumed as
video-mode panel. This leads to wrong video-mode
validations and false errors. Fix the check to get the
panel mode only from the primary display.

Change-Id: I3da3076d17cfe2314118b947133cbfc55581a195
Signed-off-by: Veera Sundaram Sankaran <veeras@codeaurora.org>