drm/i915: Don't get hw state from DVO chip unless DVO is enabled

Certain DVO chips (ns2501 for example) don't like to be accessed unless
the PLL is running. Simply skip the DVO get_hw_state if the DVO port
is disabled.

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