drm/fb-helper: No need to cache rotation and sw_rotations

Getting rotation info is cheap so we can do it on demand.

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

Cc: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Noralf Trønnes <noralf@tronnes.org>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Maxime Ripard <maxime.ripard@bootlin.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190506180139.6913-4-noralf@tronnes.org