disp: msm: dp: update checksum for edid corruption test 4.2.2.6

The DRM framework has added support to check for and calculate the
correct checksum for test cases in which the EDID is corrupted e.g.
test 4.2.2.6. Update the implementation to use the correct checksum
that is available as a field in the DP connector structure.

Change-Id: Ia9f65926e2c0fb3444aa87a4ac97850fbb4d70f3
Signed-off-by: Tatenda Chipeperekwa <tatendac@codeaurora.org>