spirv: Fix propagation of OpVariable access flags

spirv: Fix propagation of OpVariable access flags

After the decorations of a variable are evaluated, propagate the
access flag to the associated vtn_pointer.  This was done when
creating the pointer but at that point there was no access flags for
the variable.

Inline the pointer creation to make this point clearer, in isolation
the helper made the impression that the value was being propagated.

Issue found by Ken.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4620>