drm/i915: Generate i915_params {} using a macro

I want to print the struct from the error state and so would like to use
the existing struct definition as the template ala DEV_INFO*

v2: Use MEMBER() rather than p().

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Acked-by: Jani Nikula <jani.nikula@intel.com>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/20170206213608.31328-1-chris@chris-wilson.co.uk