We can implement limited RC6 counter wrap-around protection under the
assumption that clients will be reading this value more frequently than
the wrap period on a given platform.

With the typical wrap-around period being ~90 minutes, even with the
exception of Baytrail which wraps every 13 seconds, this sounds like a
reasonable assumption.

Implementation works by storing a 64-bit software copy of a hardware RC6
counter, along with the previous HW counter snapshot. This enables it to
detect wrap is polled frequently enough and keep the software copy
monotonically incrementing.

v2:
 * Missed GEN6_GT_GFX_RC6_LOCKED when considering slot sizing and
   indexing.
 * Fixed off-by-one in wrap-around handling. (Chris Wilson)

v3:
 * Simplify index checking by using unsigned int. (Chris Wilson)
 * Expand the comment to explain why indexing works.

v4:
 * Use __int128 if supported.

v5:
 * Use mul_u64_u32_div. (Chris Wilson)
Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=94852
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk> # v3
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20180208160036.29919-1-tvrtko.ursulin@linux.intel.comAcked-by: Ville Syrjälä <ville.syrjala@linux.intel.com>