meta: move vertex array enables for mipmap generation

meta: move vertex array enables for mipmap generation

Before, on the second call to GenerateMipmap we were enabling two
vertex arrays for the current vertex array object, rather than
the private generate-mipmap vertex array object.  This caused
things to blow up elsewhere.

This patch moves the array enables into the block where the
generate-mipmap vertex array object is created, as we do in
the setup_ff_generate_mipmap() function.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=60518
NOTE: This is a candidate for the stable branches.

Tested-by: core13@gmx.net
Reviewed-by: José Fonseca <jfonseca@vmware.com>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>