intel: Add support for ARB_sampler_objects.

intel: Add support for ARB_sampler_objects.

This extension support consists of replacing
"gl_texture_obj->Sampler." with "_mesa_get_samplerobj(ctx, unit)->".
One instance of referencing the texture's base sampler remains in the
initial miptree allocation, where I'm not sure we have a clear
association with any texture unit.

Tested with piglit ARB_sampler_objects/sampler-objects.

Reviewed-by: Brian Paul <brianp@vmware.com>