drm/i915/gt: Expose more parameters for emitting writes into the ring

Add another lower level to emit_ggtt_write so that the GGTT nature of
the write is not hardcoded into the emitter.

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/20201102221057.29626-1-chris@chris-wilson.co.uk