glsl: use enum glsl_interface_packing in more places. (v2)

Although the glsl_types.h stores this in a bitfield,
we should hide that from everyone else. Hide the cast
in an accessor method and use the enum everywhere.

This makes things a bit nicer in gdb, and improves type
safety.

v2: fix a few pieces of interface I missed that caused some
piglit regressions.

Signed-off-by: Dave Airlie <airlied@redhat.com>
Reviewed-by: Ilia Mirkin <imirkin@alum.mit.edu>

src/compiler/glsl/opt_dead_code.cpp

@@ -144,7 +144,7 @@ do_dead_code(exec_list *instructions, bool uniform_locations_assigned)
              * layouts, do not eliminate it.
              */
             if (entry->var->is_in_buffer_block()) {
-               if (entry->var->get_interface_type()->interface_packing !=
+               if (entry->var->get_interface_type_packing() !=
                    GLSL_INTERFACE_PACKING_PACKED)
                   continue;
             }