drm/print: move enum drm_debug_category etc. earlier in drm_print.h

Avoid forward declarations in subsequent changes, but separate this
movement to an independent change.

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Luca Coelho <luciano.coelho@intel.com>
Acked-by: Maxime Ripard <mripard@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/9d105014e3c90af13a874745d768212347f68283.1705410327.git.jani.nikula@intel.com