drm/i915: Build request info on stack before printk

printk unhelpfully inserts a '\n' between consecutive calls, and since
our drm_printf wrapper may be emitting info a seq_file instead,
KERN_CONT is not an option. To work with any drm_printf destination, we
need to build up the output into a temporary buf on the stack and then
feed the complete line in a single call to printk.

Fixes: b7268c5e ("drm/i915: Pack params to engine->schedule() into a struct")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180424010839.22860-1-chris@chris-wilson.co.uk