drm/i915/sdvo: Precompute HDMI infoframes

As with regular HDMI encoders, let's precompute the infoframes
(actually just AVI infoframe for the time being) with SDVO HDMI
encoders.

v2: Drop the WARN_ON() from drm_hdmi_avi_infoframe_from_display_mode()
    return since that could genuinely fail due to user asking
    for incompatible aspect ratio
v3: .compute_config() now returns int

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20190225174106.2163-7-ville.syrjala@linux.intel.com