drm/i915: Move legacy breadcrumb out of the reserved status page area

Addresses in the hardware status page below index 0x20 are reserved for use
by the hardware. The legacy breadcrumb was sitting at index 5. Move it to
index 0x21, and make sure everyone uses the defined value instead of
hard-coded constants.

Signed-off-by: Keith Packard <keithp@keithp.com>
Signed-off-by: Dave Airlie <airlied@linux.ie>