drm/msm/sde: add check for panel dead during ping pong timeout

Currently during ping pong timeout case, before triggering
recovery there is a check to see if panel is sending TE
properly or not. But there can be cases where ESD recovery
is already underway and hence the panel TE check function
will early return. In that case panel TE check will pass
and driver will hit panic for platforms where recovery
event is not defined. This change fixes the same by adding
additional check for panel dead flag.

Change-Id: I1c899890a7dc0d20ad46e694a1e2ce871300b948
Signed-off-by: Sandeep Panda <spanda@codeaurora.org>