drm/fb-helper: Remove drm_fb_helper_crtc->{x, y, desired_mode}

The values are already present in the modeset.

This is done in preparation for the removal of struct drm_fb_helper_crtc.

Signed-off-by: Noralf Trønnes <noralf@tronnes.org>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Maxime Ripard <maxime.ripard@bootlin.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190506180139.6913-5-noralf@tronnes.org