drm: bridge: it66121: Fix invalid connector dereference

Fix the NULL pointer dereference when no monitor is connected, and the
sound card is opened from userspace.

Instead return an empty buffer (of zeroes) as the EDID information to
the sound framework if there is no connector attached.

Fixes: e0fd83db

 ("drm: bridge: it66121: Add audio support")
Reported-by: Nishanth Menon <nm@ti.com>
Closes: https://lore.kernel.org/all/20230825105849.crhon42qndxqif4i@gondola/
Reviewed-by: Helen Koike <helen.koike@collabora.com>
Signed-off-by: Jai Luthra <j-luthra@ti.com>
Tested-by: Nishanth Menon <nm@ti.com>
Reviewed-by: Aradhya Bhatia <a-bhatia1@ti.com>
Signed-off-by: Robert Foss <rfoss@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20230901-it66121_edid-v2-1-aa59605336b9@ti.com