st/mesa: add support for lowering fp64/int64 for nir drivers

st/mesa: add support for lowering fp64/int64 for nir drivers

This might enough for iris and possible r600 (when it gets NIR)

This appears to work for iris.

v2:
 * change cap return so DOUBLES == 2 means sw emu

v3:
 * Refactor using int64/doubles lowering options which were added
   into nir options
 * Remove DOUBLES == 2 added in v2

[jordan: Remove "2" value on PIPE_CAP_DOUBLES]
[jordan: Use lowering options added to nir options]
Signed-off-by: Jordan Justen <jordan.l.justen@intel.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Acked-by: Jason Ekstrand <jason@jlekstrand.net>