drm/i915/sdvo: Fix handling if zero hbuf size

drm/i915/sdvo: Fix handling if zero hbuf size

The spec says:
"A value of 0 indicates that this buffer does not exist"
So we should not convert a hbuf_size of 0 into 1.

Also pull the relevant code into a helper to avoid making the
same mistake multiple times.

And while at it fix the debug prints to not say "hbuf_len" twice.

v2: s/%i/%u/ in the debug (Imre)

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190619180312.31817-1-ville.syrjala@linux.intel.com
Reviewed-by: Imre Deak <imre.deak@intel.com>