OMAPDSS: DISPC: set irq_safe for runtime PM

We have a bug with omapdrm, where omapdrm calls dispc's pm_runtime
function in atomic context, and dispc's pm_runtime is not marked as
irq_safe:

BUG: sleeping function called from invalid context at drivers/base/power/runtime.c:952

Dispc's runtime PM callbacks are irq safe, so we can just set the
irq_safe flag to fix the issue.

However, in the long term, I'd rather have omapdrm manage the runtime PM
calls in a better way. Calling get/put for every small operation that
touches the dispc registers is very inefficient. It'd be better and
cleaner to have clear "in-use" and "not-in-use" states for dispc, so
that we don't need to do register context restore for small operations,
only to turn dispc off right afterwards.

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