OMAPDSS: DISPC: cleanup lcd/digit enable/disable

We currently have a single function to enable and disable the manager
output for LCD and DIGIT. The functions are a bit complex, as handling
both enable and disable require some extra steps to ensure that the
output is enabled or disabled properly without errors before exiting the
function.

The code can be made simpler to understand by splitting the functions
into separate enable and disable functions. We'll also clean up the
comments and some parameter names at the same time.

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>