drm/i2c: tda998x: use drm_hdmi_avi_infoframe_from_display_mode()

Make use of the DRM HDMI AVI infoframe helper to construct the AVI
infoframe, rather than coding this up ourselves.  This allows DRM
to supply proper aspect ratio information derived from the DRM
display mode structure.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>