drm/i915: Move verify_wm_state() to heap

The stack usage exceeded 1024 bytes prompting warnings on conservative
setups, so move the temporary allocation for HW readback onto the heap.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190219122215.8941-1-chris@chris-wilson.co.uk