arm64: filmgrain: Do the right amount of gathers for subsampled fguv
arm64: filmgrain: Do the right amount of gathers for subsampled fguv

Previously we did 32 gathers even though only 16 are
needed.

Before:                          Cortex A53      A72      A73   Apple M1
fguv_32x32xn_8bpc_420_csfl0_neon:    5352.1   3985.0   4068.9   8.3
fguv_32x32xn_8bpc_420_csfl1_neon:    4738.2   3297.8   3633.0   8.2
fguv_32x32xn_8bpc_422_csfl0_neon:    5386.0   4036.8   4093.5   8.3
fguv_32x32xn_8bpc_422_csfl1_neon:    4779.9   3392.6   3641.6   8.2
fguv_32x32xn_8bpc_444_csfl0_neon:    3068.4   2422.0   2436.5   4.9
fguv_32x32xn_8bpc_444_csfl1_neon:    2558.3   1908.4   1926.6   4.4
After:
fguv_32x32xn_8bpc_420_csfl0_neon:    4330.4   3118.5   3224.6   5.3
fguv_32x32xn_8bpc_420_csfl1_neon:    3731.8   2416.9   2619.6   4.7
fguv_32x32xn_8bpc_422_csfl0_neon:    4364.7   3129.3   3247.6   5.4
fguv_32x32xn_8bpc_422_csfl1_neon:    3762.5   2450.2   2661.8   4.7
fguv_32x32xn_8bpc_444_csfl0_neon:    3075.1   2376.4   2429.4   4.9
fguv_32x32xn_8bpc_444_csfl1_neon:    2564.5   1865.9   1952.8   4.4

videolan/dav1d