[AMDGPU] Fix gfx9-4-generic LDS allocation granule (#235163)
`gfx9-4-generic` paired gfx950's 1280-byte allocation granule with
gfx942's 64 KiB LDS capacity.
Use gfx942's 512-byte granule, which matches the capacity.
Refer to ISA Doc Section 3.6.5
Fixes LCOMPILER-3031.