drm/i915: Extract vlv_prepare_pll

With this all hw writes are also gone from the ->crtc_mode_set hook on
vlv. I wondered whether we should track more of the pll state in the
pipe config, but otoh as long as we don't have shared plls that's not
really useful - the cross-checking of the port clock should be
sufficient.

While at it also de-magic some of the pipe checks, this has been
irking me since a long time.

Whit this vlv is now ready for runtime PM on dpms. If we'd have
runtime PM support in general ...

Reviewed-by: Shobhit Kumar <shobhit.kumar@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>