arm/aarch64: vp9lpf: Keep the comparison to E within 8 bit

The theoretical maximum value of E is 193, so we can just
saturate the addition to 255.

Before:                     Cortex A7      A8      A9     A53  A53/AArch64
vp9_loop_filter_v_4_8_neon:     143.0   127.7   114.8    88.0         87.7
vp9_loop_filter_v_8_8_neon:     241.0   197.2   173.7   140.0        136.7
vp9_loop_filter_v_16_8_neon:    497.0   419.5   379.7   293.0        275.7
vp9_loop_filter_v_16_16_neon:   965.2   818.7   731.4   579.0        452.0
After:
vp9_loop_filter_v_4_8_neon:     136.0   125.7   112.6    84.0         83.0
vp9_loop_filter_v_8_8_neon:     234.0   195.5   171.5   136.0        133.7
vp9_loop_filter_v_16_8_neon:    490.0   417.5   377.7   289.0        271.0
vp9_loop_filter_v_16_16_neon:   951.2   814.7   732.3   571.0        446.7

Signed-off-by: Martin Storsjö <martin@martin.st>