aco: combine MRTZ (depth, stencil, sample mask) exports

aco: combine MRTZ (depth, stencil, sample mask) exports

Instead of emitting up to 3 for each different components (depth,
stencil and sample mask). This is needed to fix a hw bug on GFX6.

Totals from affected shaders:
SGPRS: 34728 -> 35056 (0.94 %)
VGPRS: 26440 -> 26476 (0.14 %)
Spilled SGPRs: 0 -> 0 (0.00 %)
Spilled VGPRs: 0 -> 0 (0.00 %)
Private memory VGPRs: 0 -> 0 (0.00 %)
Scratch size: 0 -> 0 (0.00 %) dwords per thread
Code Size: 1346088 -> 1344180 (-0.14 %) bytes
LDS: 0 -> 0 (0.00 %) blocks
Max Waves: 3922 -> 3915 (-0.18 %)
Wait states: 0 -> 0 (0.00 %)

Signed-off-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Reviewed-by: Rhys Perry <pendingchaos02@gmail.com>
Tested-by: Marge Bot <https://gitlab.freedesktop.org/mesa/mesa/merge_requests/3538>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/merge_requests/3538>