msm_fb: display: check FB_ACTIVATE_VBL bit only

FB_ACTIVATE_VBL bit is set to enable TE logic at pan_display
and unset to disable TE logic at pan_display for smart panel.
This patch checks for FB_ACTIVATE_VBL bit set only instead of
value.

CRs-fixed: 433399
Change-Id: I6fc913c6752fcca6a27c783c45ab318065217973
Signed-off-by: Kuogee Hsieh <khsieh@codeaurora.org>