drm/amdgpu: don't register a dirty callback for non-atomic

Some asics still support non-atomic code paths.

Fixes: 66f99628

 ("drm/amdgpu: use dirty framebuffer helper")
Reported-by: Arthur Marsh <arthur.marsh@internode.on.net>
Reviewed-by: Hamza Mahfooz <hamza.mahfooz@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>