msm: mdss: hdmi: add missing sentinel to of_match_table

The DT of_match_table is missing the NULL entry that is needed to
terminate the table. Without this there is a risk of corrupting memory.

Change-Id: Ida223b91b00c72a6a2990653e3cb5ee693bd7e05
Signed-off-by: Mitchel Humpherys <mitchelh@codeaurora.org>