drm/i915: Show the current i915_params in debugfs/i915_capabilites

Alongside the hw capabilities, it is useful to know which of those have
been overridden by the user setting module parameters.

v2: Use __always_inline and BUILD_BUG magic

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Acked-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170206213608.31328-4-chris@chris-wilson.co.uk