drm/i915/psr: Add enable_source vfunc.

Continue on VLV PSR split with vfunc, let's also create one
for enabling source.

Also since we are touching *_enable_source functions let's
fix a comment with wrong name for vlv's one.

v2: Fix typo on commit message (DK).

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/20170907230041.22978-12-rodrigo.vivi@intel.com