drm/i915: Consolidate dispatch_execbuffer vfunc

v2: Put dispatch_execbuffer before add_request. (Chris Wilson)
v3: Fix add_request and irq_seqno_barrier for gen8+.

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>