drm/i915: Shift driver's HWSP usage out of reserved range

As of Gen6, the general purpose area of the hardware status page has shrunk and
now begins at dword 0x30.  i915 driver uses dword 0x20 to store the seqno which
is now reserved.  So shift our HWSP dwords up into the general purpose range
before this bites us.

Note that all available documentation just says this is reserved
without going into details about what it's used for.

Signed-off-by: Thomas Daniel <thomas.daniel@intel.com>
Reviewed-by: Dave Gordon <david.s.gordon@intel.com>
[danvet: Add clarification from Thomas that unfortunately Bspec is
silent on what "reserverd" precisely means.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>