[Why]
All calls into DC must be guarded by dc_lock because DC code
is not safe against multi-thread or re-entry.

[How]
Hold dc_lock when calling DC interfaces to parse Freesync HDMI.
Reviewed-by: Roman Li <Roman.Li@amd.com>
Acked-by: Alan Liu <HaoPing.Liu@amd.com>
Signed-off-by: Stylon Wang <stylon.wang@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>