drm/i915: Use fb modifiers in intel_check_cursor_plane

Also drop the mutex since with universal planes there is always a
proper framebuffer around which wraps the underlying bo. Which means
tiling is locked down. This was different in the old code which
directly took gem handles. The looking though was always cargo-cult
since races where not prevented in any way.

v2: Unconditionally enforce untiled, because cursors are always
untiled. The check for physical or gtt cursor is irrelevant. Also
clarify the commit message a bit

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com> (v1)
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>