drm/amdkfd: Handle drain retry fault race with XNACK mode change

Application could change XNACK enabled to disabled while KFD is draining
stale retry fault, therefore the check for whether to drain retry faults
must be before the check for whether xnack_enabled, to avoid report
incorrect vm fault after application changes XNACK mode.

Signed-off-by: Philip Yang <Philip.Yang@amd.com>
Reviewed-by: Felix Kuehling <Felix.Kuehling@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>