drm/xe/display: Rename GEN13_DISPLAY

There is no such thing as GEN13 display. With the IP disaggregation the
"gen" stopped being used. The display IP used in DG2 should be using
XE_LPD as short name. For Meterlake, it's XE_LPDP, so go ahead and
define that macro for it.

Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Link: https://lore.kernel.org/r/20230302013411.3262608-7-lucas.demarchi@intel.com