glsl: Eliminate ir_variable::data.atomic.buffer_index

glsl: Eliminate ir_variable::data.atomic.buffer_index

Just use ir_variable::data.binding... because that's the where the
binding is stored for everything else that can use layout(binding=).

Valgrind massif results for a trimmed apitrace of dota2:

                  n        time(i)         total(B)   useful-heap(B) extra-heap(B)    stacks(B)
Before (32-bit): 50 40,564,927,443       69,185,408       63,683,871     5,501,537            0
After  (32-bit): 74 40,580,119,657       69,186,544       63,506,327     5,680,217            0

Before (64-bit): 59 36,822,048,449       96,526,888       89,113,000     7,413,888            0
After  (64-bit): 89 36,822,971,897       96,526,616       88,735,296     7,791,320            0

A real savings of 173KiB on 32-bit and 368KiB on 64-bit.

Signed-off-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>