drm/amdgpu: use WARN_ON_ONCE instead of BUG_ON in the SA

Crashing the system doesn't helps at all. Also properly return
-EINVAL if size or alignment are outside valid ranges.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>