drm/i915: Use snprintf to avoid line-break when pretty-printing engines

When printing the execlist ports, we first print the ELSP header then
follow it with the pretty-printed request. Since switching to
drm_printer and show the output via printk, it automatically appends a
newline to each call (unlike the old seq_printf output). To avoid the
unwanted line break, construct the ELSP request header in a temporary
buffer.

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/20171208012303.25504-1-chris@chris-wilson.co.uk