fbdev: msm: Remove CPU sync in dma buf unmap path

During overlay_cleanup, the dma_buf_unmap_attachment api
is taking more than 1.5ms while doing the cpu_sync ops.
Use SKIP_CPU_SYNC flag to avoid this delay.

Change-Id: I3291ea54bf84285ef00d7357795c848f8e90df81
Signed-off-by: Nirmal Abraham <nabrah@codeaurora.org>