msm: mdss: fix cwb output buffer memory leak

Currently, cwb buffers are allocated and mapped during
commit but not unmapped, leaving the kernel refcounts
intact which would eventually cause memory issues. Fix it
by adding the buffer to a cleanup queue after mapping and
unmap/free the buffer in the work queue after cwb intr done.

Change-Id: I2bb41a4fadc58ecedb1fafacf6821ce6376b1b4d
Signed-off-by: Prabhanjan Kandula <pkandula@codeaurora.org>