drm/i915: make i915_fixed.h self-contained

Add the minimal includes/declarations to make the header self-contained,
and ensure it stays that way.
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/20190626144020.2155-4-jani.nikula@intel.com

drivers/gpu/drm/i915/Makefile.header-test

@@ -6,6 +6,7 @@ header_test := \
 	i915_active_types.h \
 	i915_debugfs.h \
 	i915_drv.h \
+	i915_fixed.h \
 	i915_gem_gtt.h \
 	i915_irq.h \
 	i915_params.h \

drivers/gpu/drm/i915/i915_fixed.h

@@ -6,6 +6,11 @@
 #ifndef _I915_FIXED_H_
 #define _I915_FIXED_H_
 
+#include <linux/bug.h>
+#include <linux/kernel.h>
+#include <linux/math64.h>
+#include <linux/types.h>
+
 typedef struct {
 	u32 val;
 } uint_fixed_16_16_t;