drm/i915/tracepoints: Tidy i915_gem_request_wait_begin

Provide the same information as the other request event classes.

v2: Pass in flags so we can properly report the blocking status.
    (Chris Wilson)

v3: Log hex with 0x prefix for clarity.

v4: Derive blocking status from flags. (Chris Wilson)

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>