drm/i915: BIOS and power context stolen mem handling for VLV v7

But we need to get the right stolen base and make pre-allocated objects
for BIOS stuff so we don't clobber it.  If the BIOS hasn't allocated a
power context, we allocate one here too, from stolen space as required
by the docs.

v2: fix stolen to phys if ladder (Ben)
    keep BIOS reserved space out of allocator altogether (Ben)
v3: fix mask of stolen base (Ben)
v4: clean up preallocated object on unload (Ben)
    don't zero reg on unload (Jesse)
    fix mask harder (Jesse)
v5: use unref for freeing stolen bits (Chris)
    move alloc/free to intel_pm.c (Chris)
v6: NULL pctx at disable time so error paths work (Ben)
v7: use correct PCI device for config read (Jesse)

Reviewed-by: Ben Widawsky <benjamin.widawsky@intel.com>
Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>