drm/i915/sdvo: Fix AVI infoframe TX rate readout

drm/i915/sdvo: Fix AVI infoframe TX rate readout

The AVI infoframe readout code currently issues a
SDVO_CMD_GET_HBUF_TXRATE before SDVO_CMD_SET_HBUF_INDEX, which is
not the correct order for these two operations. So far this wasn't
a problem since we left the index pointing at the AVI infoframe
buffer at the end of the modeset. However once we start to write
to other buffers (namely ELD) that is no longer going to be true.
Fix up the order so that we always read out the TX rate for the
correct buffer.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190409144054.24561-2-ville.syrjala@linux.intel.com
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>