drm/i915: Introduce encoder->get_buf_trans()

Convert the get_buf_trans() functions into an encoder vfunc.
Allows us to get rid of bunch of platform if-ladders.

v2: Handle adl-p

Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210608073603.2408-10-ville.syrjala@linux.intel.com