drm/i915/psr: Add setup VSC vfunc.

drm/i915/psr: Add setup VSC vfunc.

Continue on VLV PSR split with vfunc, let's also create
one for setting up VSC.

v2: Rebased on top of commit d2419ffc10e4 ("drm/i915: Plumb
    crtc_state to PSR enable/disable")

Cc: Daniel Vetter <[email protected]>
Cc: Dhinakaran Pandiyan <[email protected]>
Cc: Vathsala Nagaraju <[email protected]>
Signed-off-by: Rodrigo Vivi <[email protected]>
Reviewed-by: Dhinakaran Pandiyan <[email protected]>
Link: https://patchwork.freedesktop.org/patch/msgid/[email protected]