arm64: cdef: Do saturating subtractions to avoid max operations with 0
arm64: cdef: Do saturating subtractions to avoid max operations with 0

Before:                  Cortex A53     A72     A73
cdef_filter_4x4_8bpc_neon:    677.4   433.9   452.9
cdef_filter_4x8_8bpc_neon:   1255.0   815.2   841.8
cdef_filter_8x8_8bpc_neon:   2278.5  1440.0  1505.0
After:
cdef_filter_4x4_8bpc_neon:    645.5   401.9   422.5
cdef_filter_4x8_8bpc_neon:   1193.7   756.6   782.4
cdef_filter_8x8_8bpc_neon:   2162.4  1361.9  1375.6

xiph/rav1e,xiph/rav1e