drm/i915/cnl: Get RC6 working.

On CNL, individual wake rate limit was added to each engine.

GT can only go to RC6 if both Render and Media engines are
individually qualified. So we need to set their individual
wake rate limit.

+-----------------+---------------+--------------+--------------+
|                 |    GT RC6     |  Render C6   |   Media C6   |
+-----------------+---------------+--------------+--------------+
| Wake rate limit | 0xA09C[31:16] | 0xA09C[15:0] | 0xA0A0[15:0] |
+-----------------+---------------+--------------+--------------+

v2: - Tune Render and Media wake rate values according to some extra
      info I got from HW engineers. Value can be tuned, but for now
      these are the recommended values.
    - Fix typos pointed by James.

Cc: Nathan Ciobanu <nathan.d.ciobanu@intel.com>
Cc: Wayne Boyer <wayne.boyer@intel.com>
Cc: Joe Konno <joe.konno@linux.intel.com>
Cc: David Weinehall <david.weinehall@linux.intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Reviewed-by: James Ausmus <james.ausmus@intel.com>
Reviewed-by: David Weinehall <david.weinehall@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20171023224612.27208-1-rodrigo.vivi@intel.com