BugLink: http://bugs.launchpad.net/bugs/1540390

Allow the caller to specify a "prefix" string to drm_rect_debug_print()
to make it easier to see which drm_rect is being printed.
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
(cherry picked from commit c70f577a)
Signed-off-by: Timo Aaltonen <timo.aaltonen@canonical.com>
Signed-off-by: Andy Whitcroft <apw@canonical.com>