backends/drm: fix multi gpu performance with NVidia as secondary GPU
With the Nvidia driver, linear textures are external_only, so additional measures need to be taken to make the egl import path work.
Only CCBUG because that glFinish to ensure synchronization at least somewhat works has quite a heavy impact on performance - on my laptop it waits for up to 3ms, and when enough gets rendered at once, the display drops from 100Hz to 50Hz because we're missing every second frame.
This is still a big improvement over the status quo though, and without the NVidia driver supporting EGL_ANDROID_native_fence_sync we can't do a lot better at the moment.