drm/i915: Consolidate plane handler vtables

All of the previous refactoring/consolidation of plane code has resulted
in intel_primary_plane_funcs, intel_cursor_plane_funcs, and
intel_sprite_plane_funcs being identical.  Replace all of these with a
single 'intel_plane_funcs' vtable for simplicity.

Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Reviewed-by: Ander Conselvan de Oliveira <conselvan2@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>