drm/i915: use drm_debug_enabled() to check for debug categories

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

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/e94fe4977c5b8cac68556318be81f8e422e973fd.1572258936.git.jani.nikula@intel.com