xlib: use _mesa_generate_mipmap() for mipmap generation, not meta

xlib: use _mesa_generate_mipmap() for mipmap generation, not meta

The swrast fragment program interpreter has trouble computing the
right texture LOD because it doesn't have easy access to input
derivatives.  This causes the GLSL-based meta generate mipmap code
to fetch texels from the wrong mipmap level.

One possible fix would be to set the GL_TEXTURE_MIN/MAX_LOD parameters
to limit sampling from the right level.  But let's just use the
_mesa_generate_mipmap() fallback since it's a lot faster than using
the fragment shader interpreter.

Fixes http://bugs.freedesktop.org/show_bug.cgi?id=54240

Note: This is a candidate for the 9.0 branch.

Reviewed-by: José Fonseca <jfonseca@vmware.com>