drm/i915/psr: Re-org Activate after enable

Let's move the activation calls together after enable is done.

No real functional change should be expected here. Just an attempt
to get it clear when we are really activating PSR after enabling it.

v2: Add braces on if/else because commit message there is too long
    as suggested by Jani.
v3: Rebased on top of commit d2419ffc ("drm/i915: Plumb
    crtc_state to PSR enable/disable")

Cc: Jani Nikula <jani.nikula@intel.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
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/20170907230041.22978-9-rodrigo.vivi@intel.com