anv/formats: Use the real format for B4G4R4A4_UNORM_PACK16 on gen8

anv/formats: Use the real format for B4G4R4A4_UNORM_PACK16 on gen8

Because border color is handled pre-swizzle, when we move the alpha
channel around in the format, the OPAQUE_BLACK border colors don't work
correctly on B4G4R4A4_UNORM_PACK16 with the hack.  This fixes the
following Vulkan CTS tests on Broadwell:

dEQP-VK.pipeline.sampler.view_type.2d_array.format.b4g4r4a4_unorm_pack16.address_modes.all_mode_clamp_to_border_opaque_black
dEQP-VK.pipeline.sampler.view_type.1d_array.format.b4g4r4a4_unorm_pack16.address_modes.all_mode_clamp_to_border_opaque_black
dEQP-VK.pipeline.sampler.view_type.2d.format.b4g4r4a4_unorm_pack16.address_modes.all_mode_clamp_to_border_opaque_black
dEQP-VK.pipeline.sampler.view_type.1d.format.b4g4r4a4_unorm_pack16.address_modes.all_mode_clamp_to_border_opaque_black
dEQP-VK.pipeline.sampler.view_type.3d.format.b4g4r4a4_unorm_pack16.address_modes.all_mode_clamp_to_border_opaque_black

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Cc: "13.0" <mesa-stable@lists.freedesktop.org>