Move `#ifdef HAVE_DRM_ATOMIC` code into new file
There is a big chunk of code under #ifdef HAVE_DRM_ATOMIC in drm.c. It is easy to add new code inside that chunk without realizing it, leading to problems like !48 (comment 172224) .
All that code should be moved into a new source file drm-atomic.c, built conditional on what triggers HAVE_DRM_ATOMIC. When drm-atomic.c is not built, a private header needs to provide stub functions returning failures, so that we can drop almost all #ifdef HAVE_DRM_ATOMIC from drm.c.
Few special cases that still need #ifdef HAVE_DRM_ATOMIC in drm.c may remain.
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