drm/msm/dpu: Cleanup the debugfs functions

Do some debugfs cleanups from across the DPU driver. The DRM
destroy functions will do a recursive delete on the entire
debugfs node so there is no need to store dentry pointers for
the debugfs files that are persistent for the life of the
driver. This also means that the destroy functions can go
away too.

Also, use standard API functions where applicable instead of
using hand written code.

v3: No changes
v2: Add more code; most of the dpu debugfs files should be
addressed now.

Reviewed-by: Sean Paul <sean@poorly.run>
Signed-off-by: Jordan Crouse <jcrouse@codeaurora.org>
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Signed-off-by: Rob Clark <robdclark@gmail.com>