drm/amdgpu/gfx8: fix bugon.cocci warnings

 Use BUG_ON instead of a if condition followed by BUG.

Generated by: scripts/coccinelle/misc/bugon.cocci

Reviewed-by: Christian König <christian.koenig@amd.com>
Reviewd-by: Xiangliang.Yu <Xiangliang.Yu@amd.com>
CC: Xiangliang Yu <Xiangliang.Yu@amd.com>
Signed-off-by: Julia Lawall <julia.lawall@lip6.fr>
Signed-off-by: Fengguang Wu <fengguang.wu@intel.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>