glsl ubo/ssbo: Use enum to track current buffer access type

glsl ubo/ssbo: Use enum to track current buffer access type

v2:
 * Rename ssbo_get_array_length to ssbo_unsized_array_length_access (Iago)
 * Use always use this-> when referencing buffer_access_type (Iago)

Signed-off-by: Jordan Justen <jordan.l.justen@intel.com>
Reviewed-by: Iago Toral Quiroga <itoral@igalia.com>
Reviewed-by: Kristian Høgsberg <krh@bitplanet.net>