drm/i915: Improve execute_cb struct packing

Reduce the irq_work llist for attaching the callbacks to the signal for
both smaller structs (two fewer pointers!) and simpler [debug] code:

Function                                     old     new   delta
irq_execute_cb                                35      34      -1
__igt_breadcrumbs_smoketest                 1684    1682      -2
i915_request_retire                         2003    1996      -7
__i915_request_create                       1047    1040      -7
__notify_execute_cb                          135     126      -9
__i915_request_ctor                          188     178     -10
__await_execution.part.constprop             451     440     -11
igt_wait_request                             924     714    -210

One minor artifact is that the order of cb exection is reversed. No
current use cases are affected by that change.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200526112051.10229-1-chris@chris-wilson.co.uk