radv: fix a performance regression with graphics depth/stencil clears

radv: fix a performance regression with graphics depth/stencil clears

I recently changed the slow depth/stencil clear path to make sure
depth values are explicitly exported by the fragment shader. This
is actually only useful when VK_EXT_depth_range_unrestricted is
enabled.

While this path is correct, it introduced a performance regression
with Heroes of the Storm, Shadow of Mordor (Vulkan beta) and
probably more titles. This is because it prevents the hardware
to do some optimizations like discarding fragments.

This commit re-introduces the previous (a bit faster) slow
depth/stencil clear path and it selects the unrestricted path
only if VK_EXT_depth_range_unrestricted is enabled.

Closes: https://gitlab.freedesktop.org/mesa/mesa/issues/863
Signed-off-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Reviewed-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>