Revert "i965: Implement GL_PRIMITIVES_GENERATED with non-zero streams."

Revert "i965: Implement GL_PRIMITIVES_GENERATED with non-zero streams."

This reverts commit 3178d2474ae5bdd1102fb3d76a60d1d63c961ff5.

This caused GPU hangs on Ivybridge for some users and huge (80%)
performance regressions across the board on multiple platforms.

We need to find a better solution.  I've made several attempts, but none
of them have worked yet.  In the meantime, we should revert this.

Reverting it breaks GL_PRIMITIVES_GENERATED for non-zero streams, but
that's okay, since we don't expose GL_ARB_gpu_shader5 yet.

Fixes Piglit's EXT_transform_feedback/generatemipmap prims_generated
test case on Haswell.