mesa: Use location VERT_ATTRIB_GENERIC0 for vertex attribute 0

mesa: Use location VERT_ATTRIB_GENERIC0 for vertex attribute 0

In OpenGL 3.1 attribute 0 becomes non-magic, just like in
OpenGL ES 2.0. Earlier versions of OpenGL used attribute 0
exclusively for vertex position.

V2: Add a utility function _mesa_attr_zero_aliases_vertex() in
    varray.h

Fixes 4 Khronos OpenGL CTS failures:
glGetVertexAttrib
depth24_basic
depth24_precision
rgb8_rgba8_rgb

Cc: <mesa-stable@lists.freedesktop.org>
Signed-off-by: Anuj Phogat <anuj.phogat@gmail.com>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>