Allow better abstraction of the drm_debug global variable in the
future. No functional changes.

v2: move unlikely() to drm_debug_enabled()

Cc: Ben Skeggs <bskeggs@redhat.com>
Cc: nouveau@lists.freedesktop.org
Acked-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Acked-by: Sean Paul <sean@poorly.run>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/34a1e7db6eab6467c6607d9a57581d1de75d87da.1572258936.git.jani.nikula@intel.com