drm/amd/display: Don't read EDID in atomic_check

We shouldn't attempt to read EDID in atomic_check. We really shouldn't
even be modifying the connector object, or any other non-state object,
but this is a start at least.

Moving EDID cleanup to dm_dp_mst_connector_destroy from
dm_dp_destroy_mst_connector to ensure the EDID is still available for
headless mode.

Signed-off-by: Harry Wentland <harry.wentland@amd.com>
Reviewed-by: Tony Cheng <Tony.Cheng@amd.com>
Acked-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org