drm/i915: Don't cast away const from infoframe buffer

We don't modify the packed infoframe data, so we should keep the
const qualifier in place. Just pass the buffer as 'const void *'
instead of 'const uint8_t *' and we can drop the cast entirely.

v2: Do intel_sdvo_write_infoframe() as well

Reviewed-by: Damien Lespiau <damien.lespiau@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>