drm/panfrost: Fix the panfrost_mmu_map_fault_addr() error path

drm/panfrost: Fix the panfrost_mmu_map_fault_addr() error path

Make sure all bo->base.pages entries are either NULL or pointing to a
valid page before calling drm_gem_shmem_put_pages().

Reported-by: Tomeu Vizoso <tomeu.vizoso@collabora.com>
Cc: <stable@vger.kernel.org>
Fixes: 187d2929206e ("drm/panfrost: Add support for GPU heap allocations")
Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>
Reviewed-by: Steven Price <steven.price@arm.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210521093811.1018992-1-boris.brezillon@collabora.com