backends/drm: delay cursor updates with adaptive sync

While the primary plane delivers a high enough refresh rate for the cursor to be
at least usable, delay cursor updates to be in sync with the primary plane. This
avoids stutter of the content being shown on the primary plane.

Because amdgpu doesn't handle this correctly at the moment, this feature is guarded
behind the KWIN_DRM_DELAY_VRR_CURSOR_UPDATES environment variable.
See https://gitlab.freedesktop.org/drm/amd/-/issues/2186 for more details on that.

(cherry picked from commit 039fd39e)