drm/i915: Enable DVO 2x clock around DVO encoder init

ns2501 requires the DVO 2x clock before it will respond to i2c access,
so make sure the clock is enabled when we try to initialize the encoder.

Fixes the display getting lost on module reload on my Fujitsu/Siemens
Lifebook S6010.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>