drm/simpledrm: Use %p4cc to print 4CC format

Replace use of struct drm_format_name_buf with %p4cc for printing
4CC formats.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210516121315.30321-3-tzimmermann@suse.de