drm/i915: Drop stealing of bits from i915_sw_fence function pointer

Rather than stealing bits from i915_sw_fence function pointer use
separate fields for function pointer and flags. If using two different
fields, the 4 byte alignment for the i915_sw_fence function pointer can
also be dropped.

v2:
 (CI)
  - Set new function field rather than flags in __i915_sw_fence_init
v3:
 (Tvrtko)
  - Remove BUG_ON(!fence->flags) in reinit as that will now blow up
  - Only define fence->flags if CONFIG_DRM_I915_SW_FENCE_CHECK_DAG is
    defined
v4:
  - Rebase, resend for CI

Signed-off-by: Matthew Brost <matthew.brost@intel.com>
Acked-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Alan Previn <alan.previn.teres.alexis@intel.com>
Signed-off-by: John Harrison <John.C.Harrison@Intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20211116194929.10211-1-matthew.brost@intel.com