drm/i915: Subsume intel_ctx_submit_request in to drm_i915_gem_request

Move all remaining elements that were unique to execlists queue items
in to the associated request.

Issue: VIZ-4274

v2: Rebase. Fixed issue of overzealous freeing of request.
v3: Removed re-addition of cleanup work queue (found by Daniel Vetter)
v4: Rebase.
v5: Actual removal of intel_ctx_submit_request. Update both tail and postfix
pointer in __i915_add_request (found by Thomas Daniel)
v6: Removed unrelated changes

Signed-off-by: Nick Hoath <nicholas.hoath@intel.com>
Reviewed-by: Thomas Daniel <thomas.daniel@intel.com>
[danvet: Reformat comment with strange linebreaks.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>