drm/i915: make REG_BIT() and REG_GENMASK() work with variables

REG_BIT() and REG_GENMASK() were intended to work with both constant
expressions and otherwise, with the former having extra compile time
checks for the bit ranges. Incredibly, the result of
__builtin_constant_p() is not an integer constant expression when given
a non-constant expression, leading to errors in BUILD_BUG_ON_ZERO().

Replace __builtin_constant_p() with the __is_constexpr() magic spell.
Reported-by: Ville Syrjala <ville.syrjala@linux.intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190524185253.1088-1-jani.nikula@intel.com

drivers/gpu/drm/i915/i915_reg.h

@@ -126,7 +126,7 @@
  */
 #define REG_BIT(__n)							\
 	((u32)(BIT(__n) +						\
-	       BUILD_BUG_ON_ZERO(__builtin_constant_p(__n) &&		\
+	       BUILD_BUG_ON_ZERO(__is_constexpr(__n) &&		\
 				 ((__n) < 0 || (__n) > 31))))
 
 /**
@@ -140,8 +140,8 @@
  */
 #define REG_GENMASK(__high, __low)					\
 	((u32)(GENMASK(__high, __low) +					\
-	       BUILD_BUG_ON_ZERO(__builtin_constant_p(__high) &&	\
-				 __builtin_constant_p(__low) &&		\
+	       BUILD_BUG_ON_ZERO(__is_constexpr(__high) &&	\
+				 __is_constexpr(__low) &&		\
 				 ((__low) < 0 || (__high) > 31 || (__low) > (__high)))))
 
 /*