drm/amd/display: Use lockdep_assert_held() for dc_lock check

drm/amd/display: Use lockdep_assert_held() for dc_lock check

[Why]
mutex_is_locked() only tells whether *some* task holds the mutex, not
the current one, so the existing ASSERT can silently pass when the
caller violates the contract.

[How]
Use the kernel's lockdep debugging utility (include/linux/lockdep.h)
and replace ASSERT(mutex_is_locked(&dm->dc_lock)) with
lockdep_assert_held(&dm->dc_lock), which checks the current task's
held-lock stack.

Reviewed-by: Sun peng (Leo) Li <sunpeng.li@amd.com>
Signed-off-by: Ray Wu <ray.wu@amd.com>
Signed-off-by: Ivan Lipski <ivan.lipski@amd.com>
Tested-by: Dan Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>