[bugfix](CP,MLA) fix wrong slot_mapping of decode for mixed p/d batch (#6344)

### What this PR does / why we need it?
PR #5672 attempted to remove the -1 padding for duplicate tokens in the
decode slot_mapping when adapting PCP for MLAPO, and adopted a simpler
slicing approach. However, in the single-ops logic and mixed PD batches,
the decode slot_mapping did not eliminate the -1 and also shared the
slicing method, resulting in incorrect slot_mapping. This PR resolves
this issue, and the logic will be further consolidated in subsequent
refactoring PRs.

- vLLM version: v0.14.1
- vLLM main:
dc917cceb8

---------

Signed-off-by: QiuChunshuo <qiuchunshuo@huawei.com>

vllm_ascend/attention/context_parallel/mla_cp.py

@@ -79,7 +79,7 @@ class AscendMlaCPMetadataBuilder(AscendMLAMetadataBuilder):
         fast_build: bool = False,
     ) -> AscendMLAMetadata:
         metadata_cls = super().build(common_prefix_len, common_attn_metadata)
-        if self.num_prefills == 0 and self.pcp_size > 1:
+        if self.pcp_size > 1:
             self.slot_mapping[: self.num_decode_tokens] = self.slot_mapping[
                 : self.num_decode_tokens * self.pcp_size : self.pcp_size
             ]