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

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>