drm/i915: PSR: organize setup function.

psr_enabled is already by itself a setup once so let's put the W/As there and
rename old setup once to setup_vsc.

Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>