drm/i915/vrr: Latch CMRR ratio via atomic commit on debugfs write

Writing the per-CRTC "intel_vrr_target_refresh_rate" debugfs file only
updates crtc->force_cmrr, a side channel that is not tracked by the
atomic state. The requested ratio therefore does not reach the hardware
until some unrelated commit recomputes the pipe config.

Kick an internal atomic commit for the CRTC from the debugfs write and
mark the CRTC mode as changed so the pipe config is recomputed.

Note that this commit may or may not be downgraded to a fastset,
depending on whether the computed vblank value (with LRR disabled)
changes, so it is not characterised as a fastset.

If the commit fails, restore the previous numerator/denominator so that
force_cmrr does not hold on to bad values, which would otherwise make
all subsequent commits fail.

--v2:
- Commit message and code comment update (Chaitanya)
- Commit header update
- Restore to old known debugfs values when commit fails
(Chaitanya)

Signed-off-by: Mitul Golani <mitulkumar.ajitkumar.golani@intel.com>
Reviewed-by: Chaitanya Kumar Borah <chaitanya.kumar.borah@intel.com>
Signed-off-by: Suraj Kandpal <suraj.kandpal@intel.com>
Link: https://patch.msgid.link/20260730105919.3913412-8-mitulkumar.ajitkumar.golani@intel.com

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