drm/i915: Report the failure to write to the punit

drm/i915: Report the failure to write to the punit

The write to the punit may fail, so propagate the error code back to its
callers. Of particular interest are the RPS writes, so add appropriate
user error codes and logging.

v2: Add DEBUG for failed frequency changes during RPS.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Mika Kuoppala <mika.kuoppala@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170126101919.13211-1-chris@chris-wilson.co.uk
Reviewed-by: Mika Kuoppala <mika.kuoppala@intel.com>