drm/i915: s/intel_plane/plane/ in sprite init

Use a more familiar naming pattern for our variables in the sprite plane
init function.

v2: Drop the redundant 'plane' from plane_formats and num_planes_formats
    too
v3: Rebase due to ->max_stride() and ->check_plane() changes

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Stanislav Lisovskiy <stanislav.lisovskiy@intel.com> #v2
Link: https://patchwork.freedesktop.org/patch/msgid/20181005125817.22576-11-ville.syrjala@linux.intel.com