mesa/st: Tie depth clamp lowering in to the VP code

mesa/st: Tie depth clamp lowering in to the VP code

v1: implemented by Erik Faye-Lund <erik.faye-lund@collabora.com>
v2: Add handling of the ARB_clip_control depth mode
v3: Move depth_range_state to file scope and remove training zeros (Erik)
v4: - don't use the one_shader_variant property, as this is not correct (Marek)
    - also use tests on available shader stages to enable depth_clamp lowering
V5: Don't use key.st, use st directly (Marek)

Signed-off-by: Gert Wollny <gert.wollny@collabora.com>
Reviewed-by: Reviewed-by: Marek Olšák <marek.olsak@amd.com>