msm: mdss: unstage pipe only if it was ever staged

Check if pipe was staged in the mixer it's being unstaged from, before
actually unstaging and updating mixer configuration. Also, if the pipe
is being unstaged due to problems in configuration it needs to remain
unstaged in subsequent calls by clearing params_changed flag.

Change-Id: I4144a9f9d148241b83535e07ca55cdc0c9fcc4e5
Signed-off-by: Adrian Salido-Moreno <adrianm@codeaurora.org>