msm_fb: display: check BLT mode within overlay_done isr

At dsi video mode, overlay_done isr return immediately if blt mode
was not enabled. Aslo, since time between overlay_done interrupt and
dmap_done interrupt is very close, both may be serviced within same
ISR. In this case overlay_done is serviced first.

CRs-fixed: 310918
Change-Id: If8b6220c26ea8d08857f87f33609b1673238b2ec
Signed-off-by: kuogee hsieh <khsieh@codeaurora.org>