drm/file: use explicit values for enum drm_minor_type

This makes it clearer that the values cannot be changed because
they are ABI.

Signed-off-by: Simon Ser <contact@emersion.fr>
Reviewed-by: James Zhu <James.Zhu@amd.com>
Reviewed-by: Thomas Zimmermann <tzimmermann@suse.de>
Cc: Christian König <christian.koenig@amd.com>
Cc: Marek Olšák <marek.olsak@amd.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20230714104557.518457-2-contact@emersion.fr