drm/i915/selftests: Initialise request to silence a compiler

With an old (4.7.3 on 32bit) gcc, it emits a warning for

In file included from drivers/gpu/drm/i915/i915_request.c:1425:0:
drivers/gpu/drm/i915/selftests/i915_request.c: In function ‘live_nop_request’:
drivers/gpu/drm/i915/selftests/i915_request.c:380:21: error: ‘request’ may be used uninitialized in this function [-Werror=maybe-uninitialized]

Silence it by just setting it to NULL on initialisation.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180614124923.18071-1-chris@chris-wilson.co.uk