drm/i915/trace: clean up boilerplate organization

Follow the style that seems to be prevalent in kernel for undef and
define of TRACE_SYSTEM, TRACE_INCLUDE_PATH, and TRACE_INCLUDE_FILE.

There should be no changes to tracepoints.

v2: Keep TRACE_INCLUDE_PATH relative to define_trace.h (Chris)

Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Lucas De Marchi <lucas.demarchi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/0d37790ee70fb60be6f6a73d8bde2013510a7ad8.1638961423.git.jani.nikula@intel.com