radv: fix enabling sample shading with SampleID/SamplePosition

When a fragment shader includes an input variable decorated with
SampleId or SamplePosition, sample shading should be enabled
because minSampleShadingFactor is expected to be 1.0.

Cc: 19.2, 19.3 <mesa-stable@lists.freedesktop.org>
Signed-off-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Reviewed-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>
(cherry picked from commit 86a5fbfd)