drm/i915: Log HWS seqno consistently

We mix hexa- and decimal which is confusing when reading the logs. So make
the single odd one out instance decimal for consistency.

v2:
 * Do the intel_ringbuffer.c as well. (Chris Wilson)

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20180926145033.16318-1-tvrtko.ursulin@linux.intel.com