anv: advertise 8 subtexel/mipmap precision bits

So far ANV was advertising 4 bits for both subTexelPrecisionBits and
mipmapPrecisionBits. But these values were not actually verified.

But it seems the right value is actually 8 bits for both cases.

Unfortunately Intel PRM does not clarify how many bits the hardware use.
For the mipmap case, there is the following reference in PRM Volume 6
(3D Media GPGPU), specifically in LOD Computation Pseudocode:

```
Bias: S4.8
MinLod: U4.8
MaxLod: U4.8
Base: U4.1
MIPCnt: U4
SurfMinLod: U4.8
ResMinLod: U4.8
``

We have other clues, though:

- On one side, dEQP-VK.texture.explicit_lod.* tests fail when using 4
bits, but work when using 8 bits. These tests try to mimic the expected
behaviour as much real as possible, and they use the reported
subTexelPrecisionBits and mipmapPrecisionBits reported to get this.

- On the other side, the equivalent driver for Windows is reporting 8
bits for both elements. Not sure if they got to verify it from the PRM
or from a diffent source.

CC: Jason Ekstrand <[email protected]>
CC: Lionel Landwerlin <[email protected]>
Reviewed-by: Lionel Landwerlin <[email protected]>

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