i965/vec4: Port Gen8 SET_VERTEX_COUNT handling to vec4_generator.

i965/vec4: Port Gen8 SET_VERTEX_COUNT handling to vec4_generator.

Broadwell requires the number of vertices written by the geometry shader
to be specified in a separate register, as part of the terminating
message's payload.

This also means GS_OPCODE_THREAD_END needs to increment mlen.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Chris Forbes <chrisf@ijw.co.nz>
Reviewed-by: Matt Turner <mattst88@gmail.com>