drm/tilcdc: Add atomic mode config funcs

drm/tilcdc: Add atomic mode config funcs

Add atomic mode config funcs. The atomic_commit implementation is a
copy-paste from drm_atomic_helper_commit(), leaving out the async
test. The similar copy-paste implementation appears to be used in many
other drivers too. The standard drm_atomic_helper_check() is used for
checking.

The drm_atomic_helper_check() can not be used in drm_mode_config_funcs
atomic_check() callback because the plane's check implementation may
update crtc state's ->mode_changed flag. Because of this the
drm_atomic_helper_check_modeset() has to be called once more after
drm_atomic_helper_check_planes() (see drm_atomic_helper_check_modeset()
documentation).

Signed-off-by: Jyri Sarha <jsarha@ti.com>