drm/print: move drm_debug variable to drm_print.[ch]

Move drm_debug variable declaration and definition to where they are
relevant and needed. No functional changes.

Acked-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Eric Engestrom <eric@engestrom.ch>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/71a566c68883b6e6c61414cd9f7c36c84015edb1.1569329774.git.jani.nikula@intel.com
diff --git a/include/drm/drm_print.h b/include/drm/drm_print.h
index 89d38d0..fc9f218 100644
--- a/include/drm/drm_print.h
+++ b/include/drm/drm_print.h
@@ -34,6 +34,8 @@
 
 #include <drm/drm.h>
 
+extern unsigned int drm_debug;
+
 /**
  * DOC: print
  *