drm/i915: Update Promotion timer for RC6 TO Mode

When using RC6 timeout mode, the timeout value
should be written to GEN6_RC6_THRESHOLD.

v2: Updated commit message. (Tom)

v3: Rebase over whitespace differences. (Daniel)

Cc: Tom O'Rourke <Tom.O'Rourke@intel.com>
Signed-off-by: Sagar Arun Kamble <sagar.a.kamble@intel.com>
Reviewed-by: Tom O'Rourke <Tom.O'Rourke@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>