gallium: add PIPE_CAP_MAX_VARYINGS

Some NVIDIA hardware can accept 128 fragment shader input components,
but only have up to 124 varying-interpolated input components. We add a
new cap to express this cleanly. For most drivers, this will have the
same value as PIPE_SHADER_CAP_MAX_INPUTS for the fragment shader.

Fixes KHR-GL45.limits.max_fragment_input_components
Conflicts resolved by Dylan

Signed-off-by: Karol Herbst <karolherbst@gmail.com>
[imirkin: rebased, improved docs/commit message]
Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>
Acked-by: Rob Clark <robdclark@gmail.com>
Acked-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Cc: 19.0 <mesa-stable@lists.freedesktop.org>
(cherry picked from commit 6010d7b8)