drm/mm: Don't WARN if drm_mm_reserve_node

drm/mm: Don't WARN if drm_mm_reserve_node

Jesse's BIOS fb reconstruction code actually relies on the -ENOSPC
return value to detect overlapping framebuffers (which the bios uses
always when lighting up more than one screen). All this fanciness
happens in intel_alloc_plane_obj in intel_display.c.

Since no one else uses this we can safely remove the WARN without
repercussions.

Reported-by: Ben Widawsky <benjamin.widawsky@intel.com>
Tested-by: Ben Widawsky <ben@bwidawsk.net>
Cc: Jesse Barnes <jbarnes@virtuousgeek.org>
Cc: Dave Airlie <airlied@redhat.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Acked-by: Dave Airlie <airlied@redhat.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>