drm/i915: Unify the sanity checks for the buf trans tables

Get rid of the "I like my random new style best" approach and unify
the handling for the DDI buf trans table sanity checks once again.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20201207203512.1718-2-ville.syrjala@linux.intel.com


Reviewed-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: José Roberto de Souza <jose.souza@intel.com>