glsl ubo/ssbo: Move common code into lower_buffer_access::setup_buffer_access

glsl ubo/ssbo: Move common code into lower_buffer_access::setup_buffer_access

This code will also be usable by the pass to lower shared variables.

Note, that *const_offset is adjusted by setup_buffer_access so it must
be initialized before calling setup_buffer_access.

v2:
 * Add comment for lower_buffer_access::setup_buffer_access

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>