[Metal] gather_qmm improvement

[nastya236]

Same as in #4567

Given #4567 I asked Claude code to do the same for gather_qmm.

qwen3.5-397b-a17b:

Format	N	Before (ms)	After (ms)	Speedup
affine	512	8.08497	6.24240	1.30×
affine	713	8.75225	6.61468	1.32×
affine	1024	9.76828	7.38171	1.32×
affine	2048	13.55017	11.41791	1.19×
affine	4123	21.36925	16.61189	1.29×
affine	8192	36.69587	28.45843	1.29×
affine	16384	65.42287	53.63210	1.22×
nvfp4	512	8.23671	6.29423	1.31×
nvfp4	713	8.81961	6.66266	1.32×
nvfp4	1024	9.88744	7.52525	1.31×
nvfp4	2048	13.53901	11.54110	1.17×
nvfp4	4123	21.34495	16.66406	1.28×
nvfp4	8192	36.87975	28.73714	1.28×
nvfp4	16384	65.41614	54.00811	1.21×
mxfp8	512	12.06362	9.16402	1.32×
mxfp8	713	12.67015	9.46002	1.34×
mxfp8	1024	13.90283	10.22280	1.36×
mxfp8	2048	19.21845	14.19822	1.35×
mxfp8	4123	28.60970	19.87559	1.44×
mxfp8	8192	47.65744	33.30466	1.43×
mxfp8	16384	80.14714	60.24049	1.33×

[zcbenz]

Nice!