drm/panfrost: Use kvfree() to free bo->sgts

Use kvfree() to free bo->sgts, because the memory is allocated with
kvmalloc_array() in panfrost_mmu_map_fault_addr().

Fixes: 187d2929

 ("drm/panfrost: Add support for GPU heap allocations")
Cc: stable@vger.kernel.org
Signed-off-by: Denis Efremov <efremov@linux.com>
Reviewed-by: Steven Price <steven.price@arm.com>
Signed-off-by: Steven Price <steven.price@arm.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200608151728.234026-1-efremov@linux.com