ppc: Improve SATD by using vec_extract

Around 1-10% speedup of intra_satd_x3 and satd_ functions by
using vec_extract instead of vec_splat and vec_ste.

Microbenchmark results:

Power 8:
satd_4x4_altivec: 148 ==> satd_4x4_altivec: 87
satd_4x8_altivec: 186 ==> satd_4x8_altivec: 128
satd_8x4_altivec: 177 ==> satd_8x4_altivec: 114
satd_8x8_altivec: 188 ==> satd_8x8_altivec: 136
satd_8x16_altivec: 300 ==> satd_8x16_altivec: 262
satd_16x8_altivec: 269 ==> satd_16x8_altivec: 228
satd_16x16_altivec: 517 ==> satd_16x16_altivec: 485
intra_satd_x3_4x4_altivec: 528 ==> intra_satd_x3_4x4_altivec: 444
intra_satd_x3_8x8c_altivec: 679 ==> intra_satd_x3_8x8c_altivec: 593
intra_satd_x3_16x16_altivec: 1815 ==> intra_satd_x3_16x16_altivec: 1724

Power 9:
satd_4x4_altivec: 131 ==> satd_4x4_altivec: 113
satd_4x8_altivec: 175 ==> satd_4x8_altivec: 155
satd_8x4_altivec: 150 ==> satd_8x4_altivec: 135
satd_8x8_altivec: 174 ==> satd_8x8_altivec: 161
satd_8x16_altivec: 290 ==> satd_8x16_altivec: 277
satd_16x8_altivec: 272 ==> satd_16x8_altivec: 270
satd_16x16_altivec: 563 ==> satd_16x16_altivec: 566
intra_satd_x3_4x4_altivec: 424 ==> intra_satd_x3_4x4_altivec: 400
intra_satd_x3_8x8c_altivec: 687 ==> intra_satd_x3_8x8c_altivec: 616
intra_satd_x3_16x16_altivec: 2047 ==> intra_satd_x3_16x16_altivec: 2062