backends/drm: Reduce repeated calls to drmModeAdd*

Aleix Pol Gonzalezrequested to merge
work/apol/cache-drmfb into master

In the drm backends, we paint onto the EglSwapchain slots. These slots have therefore always the same dmabuf coordinates and we can afford to keep it around.

Signed-off-by: Falko Becker falko.becker@mbition.io

Tested at work and on my personal laptop, seems to work fine, hard to tell if there's a big performance impact.

This is hard to measure but it certainly happens in a hot path and requires a number of blocking syscalls to succeed, so it's worth considering its inclusion.

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