Remove 'GEN' from all registers and use GENMASK whenever possible

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

+#define GEN8_PAGE_PRESENT BIT_ULL(0)
+#define GEN8_PAGE_RW BIT_ULL(1)

We should try to avoid using deprecated "GEN" terminology in the Xe
driver. And if we're only supporting Xe-based platforms and beyond,
distinctions like "gen8" and "gen12" are pretty meaningless.

Also, should some of the masks+shifts above be replaced with GENMASK and
such so that it's easier to follow and less error-prone?