anv:Use VK_EXT_separate_stencil_usage to avoid stencil shadows on gen7

anv:Use VK_EXT_separate_stencil_usage to avoid stencil shadows on gen7

Whenever stencil texturing is not required (most of the time), we can
use VK_EXT_separate_stencil_usage to only create the shadow image when
VK_IMAGE_USAGE_SAMPLED_BIT is required for stencil.  Of course, this
depends on applications to use the extension but hopefully DXVK and
similar translators are doing so and that covers most of the apps.

Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>