drm/i915: use new cursor in intel_prepare_plane_fb v2

Simplifying the code a bit.

v2: rebased

Signed-off-by: Christian König <christian.koenig@amd.com>
Acked-by: Daniel Vetter <daniel@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20211116102431.198905-5-christian.koenig@amd.com