drm/i915: Reword warning for missing cases

In some places we end up converting switch statements to a series of
if/else, particularly when introducing helper functions to handle a
group of cases. It's tempting to either leave a wrong warning (since now
we don't have a switch case anymore) or to convert to WARN(1, ...),
but we can just provide a better message and avoid the doubt when such
conversions arrise.

Introducing a warning inside i915_driver_load() just for tests we get:

[ 4535.233717] Missing case (ret == 0)
[ 4535.233868] WARNING: CPU: 1 PID: 795 at drivers/gpu/drm/i915/i915_drv.c:1341 i915_driver_load+0x42/0x10e0 [i915]

which is clear enough.

v2: remove __func__ since this is already on the warning.

Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180319173720.6974-1-lucas.demarchi@intel.com

drivers/gpu/drm/i915/i915_utils.h

@@ -40,8 +40,8 @@
 #undef WARN_ON_ONCE
 #define WARN_ON_ONCE(x) WARN_ONCE((x), "%s", "WARN_ON_ONCE(" __stringify(x) ")")
 
-#define MISSING_CASE(x) WARN(1, "Missing switch case (%lu) in %s\n", \
-			     (long)(x), __func__)
+#define MISSING_CASE(x) WARN(1, "Missing case (%s == %ld)\n", \
+			     __stringify(x), (long)(x))
 
 #if GCC_VERSION >= 70000
 #define add_overflows(A, B) \