drm/radeon: fix up dp aux tear down (v2)

Port the amdgpu fixes from Grazvydas to radeon.

v2: drop unrelated whitespace change.

bug:
https://bugs.freedesktop.org/show_bug.cgi?id=98200



Reviewed-and-Tested-by: Michel Dänzer <michel.daenzer@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>