drm/i915/psr: hsw_psr_activate.

On HSW+ the real activate of PSR is decided by the source
after certain amount of configured idle frames.

However for the driver perspective where we track psr.active
variable this function here is the actual activate one. So
let's rename it before moving to vfunc with that.

v2: Fix typo on commit message (DK).

Cc: Dhinakaran Pandiyan <dhinakaran.pandiyan@intel.com>
Cc: Vathsala Nagaraju <vathsala.nagaraju@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Reviewed-by: Dhinakaran Pandiyan <dhinakaran.pandiyan@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20170907230041.22978-4-rodrigo.vivi@intel.com