drm/tilcdc: Remove WARN_ON(!drm_modeset_is_locked(&crtc->mutex)) checks

Remove WARN_ON(!drm_modeset_is_locked(&crtc->mutex)) checks from
tilcdc_crtc_enable(), tilcdc_crtc_disable(), and
tilcdc_crtc_update_fb().

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