drm/msm: dpu: Don't store/deref pointers in trace ringbuffer

TP_printk is not synchronous, so storing pointers and then later
dereferencing them is a Bad Idea. This patch stores everything locally to
avoid display stomped memory.

Reviewed-by: Abhinav Kumar <abhinavk@codeaurora.org>
[seanpaul fixed up commit msg typo on apply]
Signed-off-by: Sean Paul <seanpaul@chromium.org>

Signed-off-by: Rob Clark <robdclark@gmail.com>