drm/amdgpu: fix check order in amdgpu_bo_move

Reorder the code to fix checking if blitting is available.

Signed-off-by: Christian König <christian.koenig@amd.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Link: https://patchwork.freedesktop.org/patch/401019/