drm/i915: Precompute HDMI infoframes

Store the infoframes in the crtc state and precompute them in
.compute_config(). While precomputing we'll also fill out the
inforames.enable bitmask appropriately.

v2: Drop the null packet stuff (Daniel)
    Add a FIXME for lspcon
v3: .compute_config() now returns int

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20190225174106.2163-5-ville.syrjala@linux.intel.com