drm/msm/sde: add debugfs nodes for underruns debug

This change adds debugfs nodes for debugging underruns,
after this change following commands are supported:
1. To enable the underrun ftrace:
	echo 1 > /d/tracing/events/sde/sde_encoder_underrun/enable
2. To enable feature that disables the ftraces once the
	underrun happens:
	echo 1 > /d/dri/0/debug/dbg_ctrl
3. To enable the panic in the device when underrun happens:
	echo 2 > /d/dri/0/debug/dbg_ctrl
4. To enable both of the above options:
	echo 3 > /d/dri/0/debug/dbg_ctrl

Change-Id: Id9f407edb0908a5f8454f08d63c356dc8f04d353
Signed-off-by: Ingrid Gallardo <ingridg@codeaurora.org>
Signed-off-by: Jayant Shekhar <jshekhar@codeaurora.org>