anv/lower_input_attachments: honor sample index parameter to subpassLoad()

anv/lower_input_attachments: honor sample index parameter to subpassLoad()

According to GL_KHR_vulkan_glsl, the signature of subpassLoad() is:

gvec4 subpassLoad(gsubpassInput   subpass);
gvec4 subpassLoad(gsubpassInputMS subpass, int sample);

So the multisampled case always receives an explicit sample index that we
should use. The current implementation was ignoring this parameter
and using gl_SampleID value instead.

Fixes:
dEQP-VK.pipeline.multisample_shader_builtin.sample_id.*

Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>
Cc: "17.0" <mesa-stable@lists.freedesktop.org>