drm/lima: handle shared irq case for lima_pp_bcast_irq_handler

On Hikey board all lima ip blocks are shared with one irq.
This patch avoids a NULL ptr deref crash on this platform
on startup. Tested with Weston and kmscube.

Signed-off-by: Peter Griffin <peter.griffin@linaro.org>
Cc: Rob Herring <robh@kernel.org>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: Qiang Yu <yuq825@gmail.com>
Signed-off-by: Qiang Yu <yuq825@gmail.com>
Link: https://patchwork.freedesktop.org/patch/msgid/1555662781-22570-7-git-send-email-peter.griffin@linaro.org