x86: correct "length_immediate" in a few cases

When first added explicitly in 3ddffba9 ("i386.md
(sse4_1_round<mode>2): Add avx512f alternative"), "*" should not have
been used for the pre-existing alternative. The attribute was plain
missing. Subsequent changes adding more alternatives then generously
extended the bogus pattern.

Apparently something similar happened to the two mmx_pblendvb_* insns.

gcc/

	* config/i386/i386.md (sse4_1_round<mode>2): Make
	"length_immediate" uniformly 1.
	* config/i386/mmx.md (mmx_pblendvb_v8qi): Likewise.
	(mmx_pblendvb_<mode>): Likewise.