drm/i915: Simplify i830 DVO 2x clock handling

Let's just always enable the DVO 2x clock on i830. This way we don't
have to track if DVO is being used or not. The spec does suggest we
should disable the clock when it isn't needed, but this does appear
to work just fine.

This removes another crtc->config usage.

v2: Split the DPLL enable sequence change to a separate patch

Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk> #v1
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190305192400.23121-2-ville.syrjala@linux.intel.com