rutabaga_gfx: fix clippy byte_char_slices in DRM format constants

A newer clippy flags `[b'R', b'8', b' ', b' ']` under byte_char_slices; the
DRM fourcc constants are the same bytes written as a dereferenced byte
string. Values unchanged.

Signed-off-by: vadik likholetov <vadikas@gmail.com>