drm/i915/sdvo: Check that we have space for the infoframe

drm/i915/sdvo: Check that we have space for the infoframe

Before we go writing the infoframe let's make sure we have
the space for it. Not that it really matters since the write
loop would just terminate early in that case.

v2: Check after the debug print and ++ (Chris)

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20190410170835.18867-1-ville.syrjala@linux.intel.com
Reviewed-by: Radhakrishna Sripada <radhakrishna.sripada@intel.com>