drm/i915: fix dvo DPLL regression

I've missed that intel_dvo_mode_set changes the dpll configuration.
Hence when I've reworked the sequence to only enable the dpll in the
crtc_enable callback in

commit 66e3d5c0
Author: Daniel Vetter <daniel.vetter@ffwll.ch>
Date:   Sun Jun 16 21:24:16 2013 +0200

    drm/i915: move i9xx dpll enabling into crtc enable function

that special DVO bit was lost. Some BSpec reading confirms that it's
only needed for DVO encoders. Section 1.5.4, "DPLL A Control Register"
for bit 30:

"2X Clock Enable. When driving In non-gang DVO modes such as a
connected flat panel or TV, a 2X" version of the clock is needed. When
not using the 2X output it should be disabled. This bit cannot be set
when driving the integrated LVDS port on devices such as Montara-GM."

Fix this regression up.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=66516


Cc: Chris Wilson <chris@chris-wilson.co.uk>
Reported-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Partially-tested-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>