backend-drm: add DRM_FORMAT_MOD_INVALID to modifier sets when no modifiers are supported

From now on, when we can't know the modifiers supported for a certain
format, we add DRM_FORMAT_MOD_INVALID to its modifier set.

There is some parts where nothing is being added an others where
DRM_FORMAT_MOD_LINEAR is being added, so fix them.

Signed-off-by: Leandro Ribeiro <leandro.ribeiro@collabora.com>