drm/i915: fix checksum write for automated test reply

DP spec requires the checksum of the last block read to be written
when replying to TEST_EDID_READ. This patch fixes the current code
to do the same.

v2: removed loop for jumping blocks and performed direct addition
as recommended by Daniel

Signed-off-by: Sivakumar Thulasimani <sivakumar.thulasimani@intel.com>
Reviewed-by: Sonika Jindal <sonika.jindal@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>