drm/i915/dp: Change icl_dsc_compute_link_config() DSC BPP iteration

drm/i915/dp: Change icl_dsc_compute_link_config() DSC BPP iteration

Instead of iterating the valid BPP array directly, switch to the same
approach as xelpd_dsc_compute_link_config(), with a separate function to
check if the DSC BPP is valid. This prepares us for unifying the
platform specific functions.

Reviewed-by: Imre Deak <imre.deak@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/bc1972391041a3ba84b3f68b9c0605ae142611e0.1738327620.git.jani.nikula@intel.com