anv: advertise 8 subpixel precision bits

On one side, when emitting 3DSTATE_SF, VertexSubPixelPrecisionSelect is
used to select between 8 bit subpixel precision (value 0) or 4 bit
subpixel precision (value 1). As this value is not set, means it is
taking the value 0, so 8 bit are used.

On the other side, in the Vulkan CTS tests, if the reference rasterizer,
which uses 8 bit precision, as it is used to check what should be the
expected value for the tests, is changed to use 4 bit as ANV was
advertising so far, some of the tests will fail.

So it seems ANV is actually using 8 bits.

v2: explicitly set 3DSTATE_SF::VertexSubPixelPrecisionSelect (Jason)

v3: use _8Bit definition as value (Jason)

v4: (by Jason)
anv: Explicitly set 3DSTATE_CLIP::VertexSubPixelPrecisionSelect

This field was added on gen8 even though there's an identically defined
one in 3DSTATE_SF.

CC: Jason Ekstrand <jason@jlekstrand.net>
CC: Kenneth Graunke <kenneth@whitecape.org>
CC: 18.3 19.0 <mesa-stable@lists.freedesktop.org>
Signed-off-by: Juan A. Suarez Romero <jasuarez@igalia.com>
Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>
(cherry picked from commit 4f917e6a)