platformsupport/scenes/opengl: fix simpledrm

With simpledrm, only linear modifiers are supported, but with llvmpipe, only implicit modifiers are supported. As it's still possible to allocate linear buffers without modififer support by using GBM_BO_USE_LINEAR, this commit fixes that problem by adding linear modififers to the supported list.

Note that simpledrm is still kinda broken on master, it looks like there's no synchronization of llvmpipe's rendering with kms (I assume that was somehow handled in eglSwapBuffers before). Maybe we need to glFinish to get usable results again, but this at least makes outputs turn on again