drm/i915: Dynamically allocate s0ix struct for VLV

This is only required for a single platform so no need to reserve the
memory on all of them.

This removes the last direct dependency of i915_drv.h on i915_reg.h
(apart from the i915_reg_t definition).

v2: drop unneeded diff, keep the vlv prefix, call functions
    unconditionally (Jani), fwd declaration of the struct (Chris)

Signed-off-by: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Cc: Imre Deak <imre.deak@intel.com>
Cc: Jani Nikula <jani.nikula@intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20190820020147.5667-1-daniele.ceraolospurio@intel.com