AMDGPU GFX11+ v_pk_fmac_f16 should not allow DPP

[jayfoad]

Currently I get:

$ llvm-mc -triple=amdgcn -mcpu=gfx1100 -show-encoding <<< "v_pk_fmac_f16 v0, v1, v2 dpp8:[7,6,5,4,3,2,1,0]"
	v_pk_fmac_f16_dpp v0, v1, v2 dpp8:[7,6,5,4,3,2,1,0] ; encoding: [0xe9,0x04,0x00,0x78,0x01,0x77,0x39,0x05]

[llvmbot]

@llvm/issue-subscribers-backend-amdgpu

Author: Jay Foad (jayfoad)

Currently I get: ```sh $ llvm-mc -triple=amdgcn -mcpu=gfx1100 -show-encoding <<< "v_pk_fmac_f16 v0, v1, v2 dpp8:[7,6,5,4,3,2,1,0]" v_pk_fmac_f16_dpp v0, v1, v2 dpp8:[7,6,5,4,3,2,1,0] ; encoding: [0xe9,0x04,0x00,0x78,0x01,0x77,0x39,0x05] ```