util: Add a simple no-op libdrm shim

Make it easier to deal with build configs that do not have libdrm.

!21570 is working towards adding kgsl support for freedreno gallium (and platform_android, egl, etc).. which made me think a simple stub libdrm interface would avoid a lot of #ifdef HAVE_LIBDRM in various place. It doesn't completely get rid of HAVE_LIBDRM, because egl/vk extensions that are drm related. But even if we can't get rid of all the #ifdef, getting rid of some of it still seems like an improvement.