[Intel-gfx] Cleanup of early quirks

[Joonas Lahtinen]

Preparatory series to de-duplicate the stolen memory calculation code
which currently resides both in early-quirks.c and i915_gem_stolen.c
files.

Open question is how to best deliver the information from the early
quirks code to the i915 driver? Adding a new E820 memory type is one
obvious option, and having a dedicated i915_early.o built-in to
receive and relay the information when i915 is built as module or
built-in.

Regards, Joonas