drm/i915: Warn in debug builds of incorrect usages of ptr_pack_bits

GEM_BUG_ON if the packed bits do not fit into the specified width.

v2: Avoid using the macro argument twice.
v3: Drop unnecessary braces. (Joonas)
Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk> (v1)
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20171103090538.14474-1-tvrtko.ursulin@linux.intel.com

drivers/gpu/drm/i915/i915_utils.h

@@ -83,8 +83,11 @@
 	(typeof(ptr))(__v & -BIT(n));					\
 })
 
-#define ptr_pack_bits(ptr, bits, n)					\
-	((typeof(ptr))((unsigned long)(ptr) | (bits)))
+#define ptr_pack_bits(ptr, bits, n) ({					\
+	unsigned long __bits = (bits);					\
+	GEM_BUG_ON(__bits & -BIT(n));					\
+	((typeof(ptr))((unsigned long)(ptr) | __bits));			\
+})
 
 #define page_mask_bits(ptr) ptr_mask_bits(ptr, PAGE_SHIFT)
 #define page_unmask_bits(ptr) ptr_unmask_bits(ptr, PAGE_SHIFT)