drm/amd/display: Write TEST_EDID_CHECKSUM_WRITE for EDID tests

Extract edid's checksum and send it back for verification if EDID_TEST
is requested.

Signed-off-by: Mikita Lipski <mikita.lipski@amd.com>
Acked-by: Aurabindo Pillai <aurabindo.pillai@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>