Remove GEN13_DISPLAY and take a look into move display stuff out to display code

https://lore.kernel.org/all/Y9hw0sSC58B32yPg@mdroper-desk1.amr.corp.intel.com/

+#define GEN13_DISPLAY \

There's no such thing as "GEN13" so this name doesn't make sense.
Probably should have been XE_LPD instead (i.e., display version 13).

In general I want to look into moving the display feature flags and
assignment of a display subtructure into the display code itself (rather
than having the top-level i915/xe drivers set this up). I.e., the main
driver would just call something like intel_display_init() and that
centralized display code would check the PCI ID or GMD_ID to identify
itself.