v3dv: lower interpolateAt functions in NIR and enable sample rate shading

The lowering will get all the interpolateAt() functions from GLSL lowered to
the corresponding intrinsics we have just implemented in the compiler backend,
which was the last piece we needed to enable the feature.

This gets us to pass all the relevant tests in:
dEQP-VK.pipeline.multisample_interpolation.*

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/6766>

src/broadcom/vulkan/v3dv_device.c

@@ -635,7 +635,7 @@ v3dv_GetPhysicalDeviceFeatures(VkPhysicalDevice physicalDevice,
       .independentBlend = false,
       .geometryShader = false,
       .tessellationShader = false,
-      .sampleRateShading = false,
+      .sampleRateShading = true,
       .dualSrcBlend = false,
       .logicOp = false,
       .multiDrawIndirect = false,

src/broadcom/vulkan/v3dv_pipeline.c

@@ -258,6 +258,7 @@ const nir_shader_compiler_options v3dv_nir_options = {
    .lower_to_scalar = true,
    .vertex_id_zero_based = false, /* FIXME: to set this to true, the intrinsic
                                    * needs to be supported */
+   .lower_interpolate_at = true,
 };
 
 const nir_shader_compiler_options *