x86: "prefix_extra" can't really be "2"

In the three remaining instances separate "prefix_0f" and "prefix_rep"
are what is wanted instead.

gcc/

	* config/i386/i386.md (rd<fsgs>base<mode>): Add "prefix_0f" and
	"prefix_rep". Drop "prefix_extra".
	(wr<fsgs>base<mode>): Likewise.
	(ptwrite<mode>): Likewise.