msm_fb: display: check dtv_pipe before unset it

Dtv_pipe is the base layer pipe for HDMI and it is
normally populated before overlay_unset() called to
free a mdp pipe. However at wireless application,
there has possibility that overlay_unset() is called
to free a pipe even before dtv_pipe is populated which
cause system crashed. Therefore check dtv_pipe before
free fore ground pipe.

CRs-fixed: 340884
Change-Id: I32b69e937f21646204c7fd7275988ee12e439655
Signed-off-by: Kuogee Hsieh <khsieh@codeaurora.org>