drm/i915/execlists: Move the GEM_BUG_ON context matches CSB later

Print out the current request/context before doing the GEM_BUG_ON, so
that we can inspect the values in the ftrace.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180221152301.9178-1-chris@chris-wilson.co.uk