drm/amdkfd: fix process reference drop on debug ioctl

Prevent dropping the KFD process reference at the end of a debug
IOCTL call where the acquired process value is an error.

Signed-off-by: Jonathan Kim <jonathan.kim@amd.com>
Reviewed-by: Felix Kuehling <felix.kuehling@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>