drm/i915: Compile-time concatenate WARN_ON macro strings

Using __stringify(x) instead of #x adds support for macros as
a parameter and compile-time concatenation reduces the runtime
overhead.

Slightly increases the .text size but should not matter.

v2:
- Define I915_STATE_WARN_ON though I915_STATE_WARN
  (Bikeshed inspiration by Chris)

v3:
- More specific commit message

v4:
- Do not directly pass arbitary string as format, instead
  guard with "%s" (Dave)

Cc: Rob Clark <robdclark@gmail.com>
Cc: Dave Gordon <david.s.gordon@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk> (v3)
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1450441647-23924-3-git-send-email-joonas.lahtinen@linux.intel.com


Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>