drm/amdgpu: fix parsing indirect register list v2

WARN_ON possible buffer overflow and avoid unnecessary dereference.

v2: change BUG_ON to WARN_ON

Signed-off-by: Evan Quan <evan.quan@amd.com>
Reviewed-by: Huang Rui <ray.huang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>