drm/i915: don't set SDVO_BORDER_ENABLE when we're HDMI

The register that controls the HDMI port can be used to control the
sDVO port. Some bits are defined only for sDVO, and SDVO_BORDER_ENABLE
is one of those: HDMI ports that can't be used in sDVO mode don't even
have this bit defined in their specifications.

Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>