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