drm/ch7006: Stop using drm_crtc_force_disable

The correct way for legacy drivers to update properties that need to
do a full modeset, is to do a full modeset.

Note that we don't need to call the drm_mode_config_internal helper
because we're not changing any of the refcounted paramters.

v2: Fixup error handling (Ville). Since the old code didn't bother
I decided to just delete it instead of adding even more code for just
error handling.

Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com> (v1)
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181217194303.14397-1-daniel.vetter@ffwll.ch