drm/i915/dp: Calculate DSC slice count based on per-slice peak throughput

Use the DSC sink device's actual per-slice peak throughput to calculate
the minimum number of required DSC slices, falling back to the
hard-coded throughput values (as suggested by the DP Standard) if the
device's reported throughput value is 0.

For now use the minimum of the two throughput values, which is ok,
potentially resulting in a higher than required minimum slice count.
This doesn't change the current way of using the same minimum throughput
value regardless of the RGB/YUV output format used.

While at it add a TODO comment for MST tiled displays to calculate the
slice count for these based on the total pixel rate of all the tiles.

v2: Use drm helpers to query the throughput caps. (Ville)
v3: Add TODO comment to account for MST tiled displays. (Ville)

Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reported-and-tested-by: Swati Sharma <swati2.sharma@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Link: https://lore.kernel.org/r/20250930182450.563016-4-imre.deak@intel.com

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