drm/radeon: disable pll sharing for DP on DCE4.1

Causes display problems.  We had already disabled
sharing for non-DP displays.

Based on a patch from:
Niels Ole Salscheider <niels_ole@salscheider-online.de>

bug:
https://bugzilla.kernel.org/show_bug.cgi?id=58121

Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org