drm/amd/display: don't expose rotation prop for cursor plane

Setting any rotation on the cursor plane is ignored by amdgpu.
Because of DCE/DCN design, it's not possible to rotate the cursor.
Instead of displaying the wrong result, stop advertising the rotation
property for the cursor plane.

Now that we check all cursor plane properties in amdgpu_dm_atomic_check,
remove the TODO.

Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Simon Ser <contact@emersion.fr>
Cc: Alex Deucher <alexander.deucher@amd.com>
Cc: Harry Wentland <hwentlan@amd.com>
Cc: Nicholas Kazlauskas <nicholas.kazlauskas@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>