drm/amd/display: Serialize is_dp_sink_present

Access to GPIO needs to be serialized. Aux transactions are already
serialized in DRM but we also need to serialize access to the GPIO pin
for purposes of DP dongle detection.

Call is_dp_sink_present through DM so we can lock correctly. This
follows the same pattern used for DPCD transactions.

Signed-off-by: Harry Wentland <harry.wentland@amd.com>
Reviewed-by: Jun Lei <Jun.Lei@amd.com>
Acked-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>