drm/amd/display: Change EDID fallback condition

[Why]
Partially valid EDIDs on MST sinks are treated the same way as broken
EDIDs or read failures and result in a fallback EDID being used instead.

[How]
If edid_status is EDID_PARTIAL_VALID, prefer to use the valid EDID
blocks instead of using a fallback EDID.

Reviewed-by: Martin Leung <Martin.Leung@amd.com>
Acked-by: Jasdeep Dhillon <jdhillon@amd.com>
Signed-off-by: Ilya Bakoulin <Ilya.Bakoulin@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>