drm/i915: fix build for CONFIG_BUG=n

If CONFIG_BUG=n __WARN_printf won't be defined leading to the below
build failure. The double underscores should have told us to steer clear
of it anyway.

drivers/gpu/drm/i915/intel_display.c: In function ‘assert_pll’:
drivers/gpu/drm/i915/intel_display.c:1027:2: error: implicit declaration
of function ‘__WARN_printf’ [-Werror=implicit-function-declaration]
  I915_STATE_WARN(cur_state != state,

Use WARN(1, ...) instead. It handles CONFIG_BUG=n gracefully and, with
the constant condition, a sane compiler should reduce it to
__WARN_printf.

This is a regression introduced by

commit e2c719b7
Author: Rob Clark <robdclark@gmail.com>
Date:   Mon Dec 15 13:56:32 2014 -0500

    drm/i915: tame the chattermouth (v2)
Reported-by: NJim Davis <jim.epost@gmail.com>
Reference: http://mid.gmane.org/CA+r1ZhgHTi7bS2irhtuSUs9aO=Br1dumN8=oAOeaMJDZ_ZhwBw@mail.gmail.com
Cc: Rob Clark <robdclark@gmail.com>
Signed-off-by: NJani Nikula <jani.nikula@intel.com>
Signed-off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/i915_drv.h

@@ -83,7 +83,7 @@
 	int __ret_warn_on = !!(condition);				\
 	if (unlikely(__ret_warn_on)) {					\
 		if (i915.verbose_state_checks)				\
-			__WARN_printf(format);				\
+			WARN(1, format);				\
 		else 							\
 			DRM_ERROR(format);				\
 	}								\
@@ -94,7 +94,7 @@
 	int __ret_warn_on = !!(condition);				\
 	if (unlikely(__ret_warn_on)) {					\
 		if (i915.verbose_state_checks)				\
-			__WARN_printf("WARN_ON(" #condition ")\n");	\
+			WARN(1, "WARN_ON(" #condition ")\n");		\
 		else 							\
 			DRM_ERROR("WARN_ON(" #condition ")\n");		\
 	}								\