i965/vs: Generalize vertex emission code in preparation for GS.

i965/vs: Generalize vertex emission code in preparation for GS.

This patch introduces a new function, vec4_visitor::emit_vertex(),
which contains the code for emitting vertices that will need to be
common between the vertex and geometry shaders.

Geometry shaders will need to use a different message header, and a
different opcode, for their URB writes, so we introduce virtual
functions emit_urb_write_header() and emit_urb_write_opcode() to take
care of the GS-specific behaviours.

Also, since vertex emission happens at the end of the VS, but in the
middle of the GS, we need to be sure to only call
emit_shader_time_end() during VS vertex emission.  We accomplish this
by moving the call to emit_shader_time_end() into the VS
implementation of emit_urb_write_opcode().

Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>