drm/amd/display: drop cursor position check in atomic test

get_cursor_position already handles the case where the cursor has
negative off-screen coordinates by not setting
dc_cursor_position.enabled.

Signed-off-by: Simon Ser <contact@emersion.fr>
Fixes: 626bf90f ("drm/amd/display: add basic atomic check for cursor plane")
Cc: Alex Deucher <alexander.deucher@amd.com>
Cc: Nicholas Kazlauskas <nicholas.kazlauskas@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>