OMAPDSS: APPLY: add mutex

The functions in apply.c, called mostly via function pointers in overlay
and overlay_manager structs, will be divided into two groups. The first
group will not sleep and can be called from interrupts, and the second
group may sleep.

The idea is that the non-sleeping functions may only change certain
settings in overlays and managers, and those settings may only affect
the particular overlay/manager. For example, set the base address of the
overlay.

The blocking functions, however, will handle more complex configuration
changes. For example, when an overlay is enabled and fifo-merge feature
is used, we need to do the enable in multiple steps, waiting in between,
and the change affects multiple overlays and managers.

This patch adds the mutex which is used in the blocking functions to
have exclusive access to overlays and overlay managers.

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