drm/amd/display: NULL pointer error during compliance test

[Why]
Calls to disable/enable stream should be guarded with dc_lock.

[How]
Add dc_lock before calling into dc_link_handle_hpd_rx_irq.

Signed-off-by: Qingqing Zhuo <qingqing.zhuo@amd.com>
Reviewed-by: Nicholas Kazlauskas <Nicholas.Kazlauskas@amd.com>
Acked-by: Eryk Brol <eryk.brol@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>