drm/i915: Remove redundant parameters from intel_engine_cmd_parser

Declutter the calling interface by reducing the parameters to the
i915_vma and associated offsets.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191211110437.4082687-2-chris@chris-wilson.co.uk