The WARN_ONCE is a bit too verbose, make it a DRM_INFO_ONCE.

While at it, add a #define for MAX_DSLP and make the message a bit more
informative.

v2: use DRM_INFO_ONCE, add MAX_DSLP, pimp the message.
Suggested-by: NChris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: NJani Nikula <jani.nikula@intel.com>
Reviewed-by: NChris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>