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

Cc: Alex Deucher <alexander.deucher@amd.com>
Cc: Christian König <christian.koenig@amd.com>
Cc: David (ChunMing) Zhou <David1.Zhou@amd.com>
Cc: amd-gfx@lists.freedesktop.org
Acked-by: Alex Deucher <alexander.deucher@amd.com>
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/e30ddbd627be1fe1c67dd007e0d36f81a094bc91.1572258936.git.jani.nikula@intel.com