drm/i915/sdvo: Check error return from intel_sdvo_get_value()

drm/i915/sdvo: Check error return from intel_sdvo_get_value()

The current code assumes that 'enhancements' won't change in case of an
error, but this isn't guaranteed. Fix things by treating any error as a
lack of the given capability.

v2:
- Remove the now redundant init of enhancements. (Ville)

Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1494408113-379-3-git-send-email-imre.deak@intel.com