drm/i915: Implement stolen reserved detection for ctg/elk

Finally managed to dig up enough hints as to where the stolen
reserved stuff lives on ctg/elk. So add the code to decode it.
This was a combination of old chipset specs, diggin up an old
elk grits release with an ctg/elk AubLoad etc.

This was only tested on an elk as I don't have a ctg here
unfortunately.

This leaves ilk as the only platform that doesn't have a way
to detect this stuff. Looking at the register contents on my
ilk, it might be that the elk way works there too, but I
can't be sure since I can't affect the amount of reserved
memory on that machine, and if I am to trust the register
contents, by default it would reserve 0 bytes.

v2: s/WARN_ON_ONCE/WARN_ON/ since it's in one time init code
    anyway (Paulo)

Cc: Paulo Zanoni <paulo.r.zanoni@intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Acked-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>