drm/i915: Fix out of tree builds

Fixes up include paths for i915_trace.h by setting additional CFLAGS
for i915_trace_points.c to include the $src directory. The required
TRACE_INCLUDE_PATH is then "."
Signed-off-by: Peter Clifton <pcjc2@cam.ac.uk>
Signed-off-by: Eric Anholt <eric@anholt.net>

drivers/gpu/drm/i915/Makefile

@@ -33,3 +33,5 @@ i915-$(CONFIG_ACPI)	+= i915_opregion.o
 i915-$(CONFIG_COMPAT)   += i915_ioc32.o
 
 obj-$(CONFIG_DRM_I915)  += i915.o
+
+CFLAGS_i915_trace_points.o := -I$(src)

drivers/gpu/drm/i915/i915_trace.h

@@ -276,5 +276,5 @@ DEFINE_EVENT(i915_ring, i915_ring_wait_end,
 
 /* This part must be outside protection */
 #undef TRACE_INCLUDE_PATH
-#define TRACE_INCLUDE_PATH ../../drivers/gpu/drm/i915
+#define TRACE_INCLUDE_PATH .
 #include <trace/define_trace.h>