drm/i915: s/gen12/tgl/ in the universal plane code

Using "gen12" in display code is not desirable. Replace
it with "tgl" to match how we talk about other platforms
in the same code.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240918144445.5716-8-ville.syrjala@linux.intel.com


Reviewed-by: Juha-Pekka Heikkila <juhapekka.heikkila@gmail.com>