drm/i915: Hoover the level>=n_entries WARN into intel_ddi_level()

All callers of intel_ddi_level() duplicate the check+WARN
to make sure the returned level is actually present in the
appropriate buf_trans table. Let's push that stuff into
intel_ddi_level() so the callers don't have to worry about it.

Reviewed-by: Imre Deak <imre.deak@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20211001130107.1746-7-ville.syrjala@linux.intel.com