backends/drm: two small fixes

Commit 1: use sync fds for waiting for buffers to become ready in the drm backend, and pass the fds to the IN_FENCE_FD drm plane properties. Should yield the exact same result as what we're doing now, but with some drivers (like amdgpu) it can prevent oversynchronization in some edge cases.

Commit 2: fix m_vrr getting set to false wrongly. Idk why that code was there, but it's definitely just causing problems now.