drm/print: rename drm_debug to __drm_debug to discourage use

drm_debug_enabled() is the way to check. __drm_debug is now reserved for
drm print code only. No functional changes.

v2: Rebase on move unlikely() to drm_debug_enabled()

Acked-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Eric Engestrom <eric@engestrom.ch>
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/840ff7292d1a39512bac2fcb1f45de9d50694bf1.1572258936.git.jani.nikula@intel.com