video/hmdi: Clear the whole incoming buffer, not just the infoframe size

If the user if this API is providing a bigger buffer than the infoframe
size, it could be for a could reason. For instance it could be because
it gives the buffer that will be written to the hardware, up to the
maximum of an infoframe size.

Instead of just zeroing up to the infoframe size, let's zero the whole
incoming buffer as those extra bytes are also used to compute the
ECC and need to be 0.

Signed-off-by: Damien Lespiau <damien.lespiau@intel.com>
Acked-by: Dave Airlie <airlied@gmail.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>