drm/i915: Do PCH and uncore init earlier

For future platforms we'll need to initialize our MMIO function pointers
even earlier. Specifically, we'll need to be able to have register
reads/writes at GTT initialization (in i915_gem_gtt_init). Similarly,
these platforms also have MMIO differences based on the PCH id, so
while moving stuff around, also move the PCH initialization.

CC: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Ben Widawsky <ben@bwidawsk.net>
[danvet: Mention the function where we need register access.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>