drm/amd/display: Support possibly NULL link for should_use_dmub_lock

drm/amd/display: Support possibly NULL link for should_use_dmub_lock

[Why]
It's possible to have a stream enabled without a link or link encoder.

There are cases where we'd still like to interlock the driver
programming from firmware programming to ensure that we don't put the
hardware in an undefined (or error) state if two programming sequences
are simultaneously executed on the same hardware blocks.

[How]
Add an explicit DC parameter to should_use_dmub_lock().

Make pointers to should_use_dmub_lock() const since it's a checker
function that shouldn't modify state.

Update the callsites to pass in DC explicitly.

Check that the link is non-NULL before deferencing and performing link
based checks.

Reviewed-by: Alvin Lee <alvin.lee2@amd.com>
Signed-off-by: Nicholas Kazlauskas <nicholas.kazlauskas@amd.com>
Signed-off-by: Roman Li <roman.li@amd.com>
Tested-by: Dan Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>