drm/i915: Broadwell expands ACTHD to 64bit

As Broadwell has an increased virtual address size, it requires more
than 32 bits to store offsets into its address space. This includes the
debug registers to track the current HEAD of the individual rings, which
may be anywhere within the per-process address spaces. In order to find
the full location, we need to read the high bits from a second register.
We then also need to expand our storage to keep track of the larger
address.

v2: Carefully read the two registers to catch wraparound between
    the reads.
v3: Use a WARN_ON rather than loop indefinitely on an unstable
    register read.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Ben Widawsky <benjamin.widawsky@intel.com>
Cc: Timo Aaltonen <tjaalton@ubuntu.com>
Cc: Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>
Reviewed-by: Ben Widawsky <ben@bwidawsk.net>
[danvet: Drop spurious hunk which conflicted.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>