drm/amd/display: Fix hubp check in set_cursor_position

Found by smatch:
drivers/gpu/drm/amd/amdgpu/../display/dc/core/dc_stream.c:298
dc_stream_set_cursor_position() error: we previously assumed 'hubp'
could be null (see line 294)

Signed-off-by: Harry Wentland <harry.wentland@amd.com>
Reviewed-by: Tony Cheng <Tony.Cheng@amd.com>
Acked-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>