radv: fix sample_mask_in loading. (v3.1)

radv: fix sample_mask_in loading. (v3.1)

This is ported from radeonsi and fixes:
dEQP-VK.pipeline.multisample_shader_builtin.sample_mask.bit_*

v2: don't call this path for radeonsi, it does it in the epilog.
use the radeonsi code path.
v3: handle NULL pCreateInfo->pMultisampleState properly (Samuel)
v3.1: set ps_iter_samples default to 1 (Bas)

Reviewed-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>
Fixes: bdcbe7c76 (radv: add sample mask input support)
Signed-off-by: Dave Airlie <airlied@redhat.com>