drm/i915: Convert engine->write_tail to operate on a request

If we rewrite the I915_WRITE_TAIL specialisation for the legacy
ringbuffer as submitting the request onto the ringbuffer, we can unify
the vfunc with both execlists and GuC in the next patch.

v2: Drop the modulus from the I915_WRITE_TAIL as it is currently being
applied in intel_ring_advance() after every command packet, and add a
comment explaining why we need the modulus at all.

Signed-off-by: Chris Wilson <[email protected]>
Reviewed-by: Joonas Lahtinen <[email protected]>
Link: http://patchwork.freedesktop.org/patch/msgid/[email protected]
Link: http://patchwork.freedesktop.org/patch/msgid/[email protected]

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