msm_fb: display: allow backlight update after first update

bl_updated is set to non zero at pan_display if unset_bl_level
is not zero. And unset_bl_level is not updated unless bl_updated
is zero. There is a situation that bl_updated stay as zero after
first pan display due to unset_bl_level is still zero which
prevents back light from being changed until next pan display
even though unset_bl_level changed. This patch break up this loop
and allow back light being changed right after first pan display.

CRs-fixed: 471983
Change-Id: I75504206853517672f968e8ea8fc6782a282e326
Signed-off-by: Kuogee Hsieh <khsieh@codeaurora.org>