msm: mdss: Reset rotator_mode of writeback mixer

The variable rotator_mode of mixer structure is used to
program writeback mixer in block mode. This structure
member must be cleared on finish.

Change-Id: I09b938def0cf1745f18e20d3bb04555209d1f269
Signed-off-by: Jayant Shekhar <jshekhar@codeaurora.org>