drm: fix/clean up buffer handling

This rectifies ioctl parameters so that allocating frame buffers no longer fails in kernel. This also avoids needlessly altering the input format (and thus adding unnecessary conversion filters).

Lastly, this cleans up the triple-buffered frame buffer handling: now we keep one picture buffer for each of the 3 front buffers, instead of switching the property of a dummy picture all the time.

Fixes #26707 (closed)