drm/amdgpu/dc: Stop updating plane->fb

We want to get rid of plane->fb on atomic drivers. Stop setting it.

Cc: Alex Deucher <alexander.deucher@amd.com>
Cc: "Christian König" <christian.koenig@amd.com>
Cc: "David (ChunMing) Zhou" <David1.Zhou@amd.com>
Cc: Harry Wentland <harry.wentland@amd.com>
Cc: amd-gfx@lists.freedesktop.org
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20180525185045.29689-8-ville.syrjala@linux.intel.com


Reviewed-by: Sinclair Yeh <syeh@vmware.com>