mdss: mdp: fix pipe/mixer stage information during free

MDSS driver stages the pipe on certain mixer but does not
update the stage index to unused after un-stagging the pipe
from kickoff. It is better that pipe free api checks the
mixer stage before setting pipe mixer null. This helps
to catch the bugs where pipe is getting staged without
commit call.

This change tries to update the pipe free API with proper
error checks. It also avoids mixer free twice from ctl
destroy to avoid error messages at each suspend/resume.

Change-Id: I6facef86b0816378d79fd17737b0f335c59ebfa6
Signed-off-by: Dhaval Patel <pdhaval@codeaurora.org>