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

This causes instability in piglit.  It's fixed in drm-next with:
515c6faf85970af529953ec137b4b6fcb3272e25
1650c14b459ff9c85767746f1ef795a780653128
214a91e6bfabaa6cbfa692df8732000aab050795
29d253553559dba919315be847f4f2cce29edd42
79867462634836ee5c39a2cdf624719feeb189bd

This reverts commit 6af0883e.

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