drm/i915: Hold rpm wakeref for modifying the global seqno

To modify the global seqno may require rewriting a few registers, which
requires us to hold the rpm wakeref. We must therefore take it around
the call to i915_gem_set_global_seqno() in debugfs, on behalf of the
user.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Michał Winiarski <michal.winiarski@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180102151235.3949-15-chris@chris-wilson.co.uk