msm: mdss: Avoid unnecessary warnings during pipe unstaging

In some cases, when the pipe is being unstaged from right mixer,
some unnecessary processing is done and warning messages are
being printed. This can be avoided by updating the right stage
information after unstaging the pipe.

Change-Id: Ibeaa1c4bee29d7ac3ae34140bea8d5b7468dbc85
Signed-off-by: Krishna Chaitanya Devarakonda <kdevarak@codeaurora.org>