[CK_TILE][FMHA] Improve precision of mxfp4 FMHA with fp6 for matrix P (#5388)
## Motivation
Improve precision of mxfp4 without performance penalties.
## Technical Details
Since performance of scale MFMAs is the same when neither A nor B is
fp8/bf8, it is possible to use fp6 x fp4 instead of fp4 x fp4 for the
second GEMM, while types of Q, K, V stay the same.
This allows to improve overall precision significantly because fp6 has
32 non-negative values used for P quantization compared to just 8 values
for fp4.
It was found that there is a compiler bug with
`__builtin_amdgcn_cvt_scalef32_2xpk16_fp6_f32` (described in
LCOMPILER-561) but a workaround seems to fix all failing instances.
## Test Plan
```
ninja test_ck_tile_fmha_fwd_mxfp4 && bin/test_ck_tile_fmha_fwd_mxfp4
```
## Test Result
The tests must pass.
## Submission Checklist
- [x] Look over the contributing guidelines at
https://github.com/ROCm/ROCm/blob/develop/CONTRIBUTING.md#pull-requests.
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