drm/i915: Fix gtt_view asserts

gcc is too smart for us and doesn't evaluate BUILD_BUG_ON()s in
unused static inlines. Collect them up in one static inline and
actually call it to make sure gcc sees it.

Cc: Chris Wilson <chris@chris-wilson.co.uk>
Suggested-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180828133723.18505-1-ville.syrjala@linux.intel.com
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>