drm/amd/display: Drop atomic_obj_lock for private obj

[Why]
New DRM versions manage locking for private objects for us, so this
is no longer needed.

This also prevents a WARN_ON from occurring when the private object is
duplicated during the forced atomic commit that occurs from the HPD
handler.

The HPD handler calls drm_modeset_lock_all before the forced commit
and if the private object is duplicated then the
DEBUG_LOCKS_WARN_ON(ww_ctx->done_acquire) warning will be triggered
since we're trying to lock something when everything should have
already been locked.

[How]
Drop the lock and let DRM manage this.

Cc: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Nicholas Kazlauskas <nicholas.kazlauskas@amd.com>
Reviewed-by: Leo Li <sunpeng.li@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>