drm/i915: Switch to unified plane cursor handling

The DRM core will translate calls to legacy cursor ioctls into universal
cursor calls automatically, so there's no need to maintain the legacy
cursor support.  This greatly simplifies the transition since we don't
have to handle reference counting differently depending on which cursor
interface was called.

The aim here is to transition to the universal plane interface with
minimal code change.  There's a lot of cleanup that can be done (e.g.,
using state stored in crtc->cursor->fb rather than intel_crtc) that is
left to future patches.

Signed-off-by: Matt Roper <matthew.d.roper@intel.com>