msm_fb: display: check base layer pipe at irq handler

There is a corner case when overlay and dmap interrupts
are fired before base layer pipe is allocated. Therefore
Check for base layer pipe before performing any operations.

Change-Id: Ib188d575044f5479672503508042c8c1e9c759c4
Signed-off-by: Kuogee Hsieh <khsieh@codeaurora.org>