drm/i915/vrr: Compute CMRR fractional timings generically

Rework the fractional CMRR computation into a generic,
transcoder-agnostic helper driven by an explicit per-CRTC debugfs
target, replacing the previous disabled, eDP-only code path. Compute
CMRR_M and CMRR_N timings based on the video mode requirements. A 1:1
ratio (denominator == 1000) means no video timing is required. Note
that the CMRR enable path is wired up separately; this patch only lays
down the generic computation.

Remove computation of mode_flags, I915_MODE_FLAG_VRR, as CMRR is
being moved to the fixed refresh rate path. Also return early from
the CMRR computation if the existing computation is already sufficient.

--v2:
- Derive video_mode locally instead of caching it in the persistent
  struct intel_crtc state (Jani, Chaitanya)
- Fix numerator unit in comment: milli-Hz, not kHz (Chaitanya)
- Fix "requirement" typo and clarify that CMRR is not yet enabled in
  the commit message (Chaitanya)
- Fix precision issue while computing the M/N ratio (Chaitanya)
- Rename multiplier_m and multiplier_n to improve readability
  (Chaitanya)
- Compute vtotal as it is required for dithering as per the algorithm
  implementation (Chaitanya)
- Replace the misleading adjusted_pixel_rate with dividend, which is
  more descriptive

--v3:
- Mention the rationale for not computing I915_MODE_FLAG_VRR
  (Chaitanya)
- Remove the redundant return statement at the end of the CMRR
  computation (Chaitanya)
- Correct cmrr_n calculation (Chaitanya)
- Add an early return if the current computation is already sufficient
  to drive the mode without CMRR (Chaitanya)
- Round up requested_refresh_rate (Validation)
- Fix integer overflow

--v4:
- Update cmrr_n calculation to avoid aggressive dithering
- Use div64_u64 to truncate to the floor value instead of
  DIV_ROUND_UP_ULL, which rounds up to the ceiling value
- Add a TODO comment to the cmrr_n calculation
- Add a condition for custom refresh rate requests
- Update the CMRR computation comment block in compute_config

--v5:
- Simplify computation
- Correct typo in commit message (Chaitanya)
- Compute early return condition (Chaitanya)

--v6:
- Simplify commit message
- Simplify video timing requirement compute function (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/20260730105919.3913412-7-mitulkumar.ajitkumar.golani@intel.com

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