Fix auto generating mipmaps for incomplete textures.

Bug 21278131

Change-Id: I1969f40ab4cb5e475c9c0a4e5f7d4238a3deb7be
Reviewed-on: https://swiftshader-review.googlesource.com/3220
Tested-by: Nicolas Capens <capn@google.com>
Reviewed-by: Alexis Hétu <sugoi@google.com>
Tested-by: Ping-Hao Wu <pinghao@google.com>
Reviewed-by: Nicolas Capens <capn@google.com>