radv: Fix float16 interpolation set up.

float16 types can have non-flat interpolation so set up the HW
correctly for that.

Fixes: 62024fa7 "radv: enable VK_KHR_16bit_storage extension / 16bit storage features"
Reviewed-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
(cherry picked from commit a1fdd4a4)
[Emil: resolve trivial conflicts]
Signed-off-by: Emil Velikov <emil.velikov@collabora.com>

Conflicts:
	src/amd/vulkan/radv_nir_to_llvm.c