drm/i915/doc: Update parallel submit doc to point to i915_drm.h

Update parallel submit doc to point to i915_drm.h

Signed-off-by: Matthew Brost <matthew.brost@intel.com>
Reviewed-by: John Harrison <John.C.Harrison@Intel.com>
Signed-off-by: John Harrison <John.C.Harrison@Intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20211014172005.27155-18-matthew.brost@intel.com