drm/i915: Simplify intel_mark_busy/idle

They use dev_priv exclusively so pass it in instead of dev
for smaller source and binary.

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: http://patchwork.freedesktop.org/patch/msgid/1461844620-35360-1-git-send-email-tvrtko.ursulin@linux.intel.com