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>