drm/amdkfd: drop process ref count when xnack disable

During svm restore pages interrupt handler, kfd_process ref count was
never dropped when xnack was disabled. Therefore, the object was never
released.

Fixes: 2383f56b ("drm/amdkfd: page table restore through svm API")
Signed-off-by: Alex Sierra <alex.sierra@amd.com>
Reviewed-by: Philip Yang <philip.yang@amd.com>
Reviewed-by: Jonathan Kim <jonathan.kim@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org