OMAPDSS: HDMI: Add locking for hdmi interface set timing functions

The hdmi interface driver exposes functions to the hdmi panel driver to
configure the interface timings maintained by the hdmi driver.

These timings(stored in hdmi.ip_data.cfg) should be protected by the hdmi lock
to ensure they are called sequentially, this is similar to how hdmi enable and
disable functions need locking.

Signed-off-by: Archit Taneja <archit@ti.com>