drm/i915/sdvo: Actually print the reason why the SDVO command failed

It's much easier to figure out why the SDVO encoder refuses to cooperate
if we can see what status we got back.

v2: Zero initialize only the first character, not the whole buffer

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk> #v1
Link: https://patchwork.freedesktop.org/patch/msgid/20190410170941.28142-1-ville.syrjala@linux.intel.com