drm/i915: Use 32bit read for BB_ADDR

The BB_ADDR register is documented to be 32bits at least since SNB.
Prior to that the high 32bits were listed as MBZ, so using a 64bit read
doesn't seem worth anything. Also the simulator doesn't like the 64bit
read. So just switch to using a 32bit read instead.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Ben Widawsky <ben@bwidawsk.net>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>