mesa: Refactor enable_vertex_array_attrib to make _mesa_enable_vertex_array_attrib

mesa: Refactor enable_vertex_array_attrib to make _mesa_enable_vertex_array_attrib

Pulls the parts of enable_vertex_array_attrib that aren't just parameter
validation out into a function that can be called from other parts of
Mesa (e.g., meta).

_mesa_enable_vertex_array_attrib can also be used to enable
fixed-function arrays.

Signed-off-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Anuj Phogat <anuj.phogat@gmail.com>
(cherry picked from commit 4a644f1caadc6b3e26b5f0ac60ac855152e38e59)