OMAPDSS: DPI: Add locking for DPI interface

The DPI interface driver currently relies on the panel driver to ensure calls
like omapdss_dpi_display_enable() and omapdss_dpi_display_disable() are executed
sequentially. Also, currently, there is no way to protect the DPI driver data.

All DPI panel drivers don't ensure this, and in general, a DPI panel driver
should use it's lock to that ensure it's own driver data and omap_dss_device
states are taken care of, and not worry about the DPI interface.

Add mutex locking in the DPI enable/disable/set_timings ops.

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