drm/drv: use enum drm_minor_type when appropriate

This makes it easier to figure out what the "type" variable can be
set to when reading the implementation of these functions.

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/20230714104557.518457-1-contact@emersion.fr