mmx: Use Loongson pinsrh instruction in pack_565

mmx: Use Loongson pinsrh instruction in pack_565

The pinsrh instruction is analogous to MMX EXT's pinsrw, except like
other Loongson vector instructions it cannot access the general purpose
registers. In the cases of other Loongson vector instructions, this is a
headache, but it is actually a good thing here. Since the instruction is
different from MMX, I've named the intrinsic loongson_insert_pi16.

 text	data	bss	dec	 hex filename
25976	1952	  0   27928	6d18 .libs/libpixman_loongson_mmi_la-pixman-mmx.o
25336	1952	  0   27288	6a98 .libs/libpixman_loongson_mmi_la-pixman-mmx.o

-and: 181
+and: 147
-dsll: 143
+dsll: 95
-dsrl: 87
+dsrl: 135
-ldc1: 523
+ldc1: 462
-lw: 767
+lw: 721
+pinsrh: 35