drm: drop DRM_DEBUG_CODE

DRM_DEBUG_CODE is currently always set, so distributions enable it. The
only reason to keep support in code is if developers wanted to disable
debug support. Sounds unlikely.

All the DRM_DEBUG() printks are still guarded by a drm_debug read. So if
its cacheline is read once, they're discarded pretty fast.. There should
hardly be any performance penalty, it's even guarded by unlikely().

Signed-off-by: David Herrmann <dh.herrmann@gmail.com>
Reviewed-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>