gallium/util: Save and restore vertex buffer state in util_gen_mipmap.

gallium/util: Save and restore vertex buffer state in util_gen_mipmap.

Calling glGenerateMipmap could overwrite vertex buffer state, leading
 to incorrect rendering or crashes depending on the Gallium driver.

This was happening on WebGL Conformance test texture-size.

Before 784dd51198433e5c299da4a7742c68d21d68d1c1 this was covered up
by redundant vertex buffer validation.

Reviewed-by: Stéphane Marchesin <marcheu@chromium.org>
Signed-off-by: Marek Olšák <maraeo@gmail.com>