i965: Virtualize vec4_visitor::emit_urb_slot().

i965: Virtualize vec4_visitor::emit_urb_slot().

This avoids a downcast of key, which won't exist in the base class soon.

I'm not a huge fan of this patch, but given that we're currently using
inheritance, this seems like the "right" way to do it.  The alternative
is to make key a void pointer in the parent class and continue
downcasting.

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