render/drm_format_set: disallow DRM_FORMAT_INVALID
render/drm_format_set: disallow DRM_FORMAT_INVALID

It doesn't make sense to add DRM_FORMAT_INVALID to a format set. Adding
an assertion allows us to safely query the format set with
DRM_FORMAT_INVALID. See [1].

[1]: https://github.com/swaywm/wlroots/pull/2021#discussion_r385839668

swaywm/wlroots,ascent12/wlroots,swaywm/wlroots,SirCmpwn/wlroots,ascent12/wlroots