drm/amdkfd: potential error pointer dereference in ioctl

The "target" either comes from kfd_create_process() which returns error
pointers on error or kfd_lookup_process_by_pid() which returns NULL on
error.  So we need to check for both types of errors.

Fixes: 0ab2d753 ("drm/amdkfd: prepare per-process debug enable and disable")
Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
Reviewed-by: Jonathan Kim <jonathan.kim@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>