msm: camera: isp: Avoid stats data corruption.

When several threads permanently, e.g., read/write the debugfs
file that triggers vfe_debugfs_statistics_read(), the content of
stats will be modified in an unintended manner, add spin_lock
to protect that.

Change-Id: I15d46498e669132ef1d1f5ad9c4305ee5fdb0da4
Signed-off-by: Ramesh V <ramev@codeaurora.org>