anv: use shadow surface for stencil input attachment on gfx7

anv: use shadow surface for stencil input attachment on gfx7

This fixes a number of tests like :
  dEQP-VK.renderpass*.suballocation.multisample.s8_uint.*
  dEQP-VK.renderpass*.suballocation.multisample.separate_stencil_usage.d24_unorm_s8_uint.*.test_stencil
  dEQP-VK.renderpass*.suballocation.multisample.d24_unorm_s8_uint.*
  dEQP-VK.renderpass*.suballocation.multisample.d32_sfloat_s8_uint.*

Because the driver asserts when generating RENDER_SURFACE_STATE with a
8 Valign value for stencil buffer (only 2 & 4 are supported).

Signed-off-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Reviewed-by: Nanley Chery <nanley.g.chery@intel.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/12670>