drm/tilcdc: cleanup runtime PM handling

Cleanup runtime PM handling. Before the patch the usage of pm_runtime
calls was inconsistent and hard to follow. After the update the
pm_runtime calls are removed from set_scanout() and called around
major operations that access the HW. After the patch the DPMS code does
not have pm_runtime_forbid/allow calls any more and
pm_runtime_irq_safe() is not set anymore.

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
[Added description to the patch]
Signed-off-by: Jyri Sarha <jsarha@ti.com>