drm/msm: add a helper function to return the EDID checksum

Display Port Link Compliance tests expect to return the checksum
of the last block of EDID data. Add a function that returns this
checksum.

Change-Id: I6f316967eecd71f7344a0bb142c641b4ccf6c35f
Signed-off-by: Padmanabhan Komanduru <pkomandu@codeaurora.org>
2 files changed