drm/tilcdc: Remove obsolete crtc helper functions

Remove obsolete crtc helper functions. These are not needed when
atomic modeset is used.

Note that the drm_crtc_helper_funcs mode_fixup() is still needed. The
crtc's check() callback can not do its job here.

The plane's check() callback needs to set drm_crtc_state's
->mode_changed to true if the pixel format for the framebuffer
changes. Because of this drm_mode_config_funcs atomic_check() callback
needs to call drm_atomic_helper_check_modeset() once more after it has
called drm_atomic_helper_check_planes(). If the fixing of the
adjusted_mode would be done in drm_crtc_helper_funcs atomic_check()
callback, it would get over written by the extra
drm_atomic_helper_check_modeset() call.

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