i965/gen6: enable GLSL 1.50, OpenGL 3.2 and GL_AMD_vertex_shader_layered

i965/gen6: enable GLSL 1.50, OpenGL 3.2 and GL_AMD_vertex_shader_layered

Geometry shaders was the only thing we needed to enable GLSL 1.50 and
OpenGL 3.2 in gen6.

v2: Layered clears do not work properly in gen6 with OpenGL 3.2. Kenneth
and Jordan realized that for this to work we also need
GL_AMD_vertex_shader_layered (which requires OpenGL 3.2, so it could not be
enabled before this patch), so we agreed to enable this together with
OpenGL 3.2 in this patch.

Signed-off-by: Samuel Iglesias Gonsalvez <siglesias@igalia.com>
Reviewed-by: Matt Turner <mattst88@gmail.com>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>
Acked-by: Kenneth Graunke <kenneth@whitecape.org>