Revert "drm/amdgpu: discard commands of killed processes"

This causes instability in piglit.  It's fixed in drm-next with:
515c6faf
1650c14b
214a91e6
29d25355
79867462

This reverts commit 6af0883e.

Signed-off-by: Alex Deucher <alexander.deucher@amd.com>