aco: use VOP2 for v_cvt_pkrtz_f16_f32 if possible

aco: use VOP2 for v_cvt_pkrtz_f16_f32 if possible

This patch also does a slight rework of export_fs_mrt_color()
to avoid setting of enabled channels which are not used.

Totals from 52404 (38.38% of 136546) affected shaders (NAVI):
SGPRs: 3097443 -> 3097435 (-0.00%)
CodeSize: 189151600 -> 188546200 (-0.32%)
Instrs: 36445061 -> 36445104 (+0.00%); split: -0.00%, +0.00%
Cycles: 1739388020 -> 1739388192 (+0.00%); split: -0.00%, +0.00%
VMEM: 21071501 -> 21071665 (+0.00%); split: +0.00%, -0.00%
SMEM: 3470983 -> 3470982 (-0.00%); split: +0.00%, -0.00%
PreSGPRs: 2058965 -> 2058962 (-0.00%)
PreVGPRs: 1860294 -> 1860295 (+0.00%)

Reviewed-by: Rhys Perry <pendingchaos02@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/6777>