Ndrm/i915/debugfs: store rotation string buffer on stack

Minimal change to nuke the static buf.

Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190107145149.10069-1-jani.nikula@intel.com