glapi: Alias VertexAttribDivisor and VertexAttribDivisorARB.

glapi: Alias VertexAttribDivisor and VertexAttribDivisorARB.

There's no reason to have separate slots in the dispatch table for
these two functions, since they are synonymous.

Note: previous to this patch, we never populated the dispatch table
slot for VertexAttribDivisor, which was ok, since it is not required
until 3.3.  After this patch, both functions will be usable provided
that the ARB_instanced_arrays extension is present.

Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>