OMAPDSS: Displays: Add locking in generic DPI panel driver

The generic DPI panel driver doesn't currently have locking to ensure that
the display states and the driver data is maintained correctly. Add mutex
locking to take care of this. Add a new get_timings driver op to override the
default get_timings op. The new driver op contains locking to ensure the correct
panel timings are seen when a DSS2 user calls device->driver->get_timings.

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