nir: Allocate nir_call_instr::params out of the nir_call itself.

nir: Allocate nir_call_instr::params out of the nir_call itself.

The lifetime of the params array needs to be match the nir_call_instr
itself.  So, allocate it using the instruction itself as the context.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Jason Ekstrand <jason.ekstrand@intel.com>