drm/i915: Extract intel_bw_check_data_rate()

Extract the data rate calculation loop out from
intel_bw_atomic_check() to make it a bit less confusing.

v2: Deal with 'bool changed'

Reviewed-by: Stanislav Lisovskiy <stanislav.lisovskiy@intel.com> #v1
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220218064039.12834-7-ville.syrjala@linux.intel.com