[CK_TILE] Optimize FMHA head-dim padded path on gfx11/gfx12
(#6156)
## Motivation
On gfx11/gfx12, FMHA forward kernels that require head-dim padding show
a large performance drop compared to the exact-head-dim path. In
practice, padded cases such as `HDIM=72` and `HDIM=80` were falling too
far off the fast path.
This PR improves padded-head-dim FMHA performance on gfx11/gfx12 while
keeping the behavior for other GPUs unchanged.
## Technical Details
- Add/scope a dedicated padded-head-dim (`qr_hpad`) FMHA forward path
for gfx11/gfx12.
- For `receipt=0`, keep support conservative and only enable the padded
fast path for vector-safe cases (`head_dim % 8 == 0`), matching the
existing assumption used on other GPUs.
- Move `v_prefetch` later only for the head-dim-padded path on
gfx11/gfx12. This reduces live ranges and removes the register-spill
behavior seen in the earlier scheduling.
- Enable the buffer-load OOB check offset trick for the padded path on
gfx11/gfx12.
## Test Plan
./build/bin/tile_example_fmha_fwd -prec=bf16 -mode={0/1} -b=1 -h=16
-d={72/80} -s={seqlen} -s_k={seqlen} -lse=0 -iperm={0/1} -operm={0/1}
## Test Result
Observed padded-head-dim performance improvements for HDIM=72/80:
- gfx11: about ~3.5x
- gfx1151: about ~2.0x
- gfx12: about ~1.3x
## Submission Checklist
- [ ] Look over the contributing guidelines at
https://github.com/ROCm/ROCm/blob/develop/CONTRIBUTING.md#pull-requests.
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