drm/i915/cdclk: Deal with 2 PPC hscale issues when calculating min CDCLK

Double the fractional part of the horizontal scale factor
for the purposes of min_cdck calculation. This bumps the min
CDCLK sufficiently to overcome some kind of 2 PPC granularity
issue. Without this CDCLK may end up being too low and we get
underruns with certain horizontal downscale factors.

The current Bspec formula calls for doubling only the
fractional part below 0.5, and rounding it down to a
unit fraction. But that formula does not result in a
sufficient CDCLK bump in a lot of cases. Empirical evidence
supports doubling the entire fractional part, so let's just
do that while we wait for further analysis from the hardware
team.

Also note that the position of the scaler output window also
seems to matter. If the output is near the left edge of the
screen then lower CDCLK is sufficient, but moving the output
window further to the right causes underruns unless CDCLK is
also bumped. Some prefill happening during hblank already?

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patch.msgid.link/20260715120926.10786-5-ville.syrjala@linux.intel.com
Reviewed-by: Nemesa Garg <nemesa.garg@intel.com>
Tested-by: Vidya Srinivas <vidya.srinivas@intel.com>

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