drm/i915/psr: Unify VSC setup functions.

VSC package is decided per eDP spec for psr1 or psr2,
and not per platform, so let's unify it and kill "skl"
func.

v2: Rebased on top of commit d2419ffc ("drm/i915: Plumb
    crtc_state to PSR enable/disable")

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/20170907230041.22978-6-rodrigo.vivi@intel.com