msm: mdss: refactor mdp debug statistics

Move debug statistics to mdp5 specific debug file and make use of
simpler seq_* calls to display MDP statistics through debugfs file.

Change-Id: I424b9c62e993390670448c9db6d0800e97c830eb
Signed-off-by: Adrian Salido-Moreno <adrianm@codeaurora.org>