drm/msm/sde: enable register dump on underrun

This change enables register capture on underrun interrupt.
The register capture and panic is only enabled if user
sets DBG_CTRL_PANIC_UNDERRUN on dbgfs control, which can be
enabled for any underrun debug

Change-Id: I059a13d3e8db7caadce06937d219175aeddbebf1
Signed-off-by: Abhijit Kulkarni <kabhijit@codeaurora.org>