turnip: Advertise 8 bit subpixel precision

turnip: Advertise 8 bit subpixel precision

Previously, turnip advertised 4-bit subpixel precision when in
practice, a6xx seems to render with 8-bit precision. This caused
dEQP-VK.renderpass2.suballocation.subpass_dependencies.late_fragment_tests.*
to fail because they compare images rendered with turnip against
ones rendered via a software reference implementation parameterized
by turnip's VkPhysicalDeviceLimits.subPixelPrecisionBits value.

Tested-by: Marge Bot <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4172>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4172>