drm/i915: Extract intel_cursor_check_surface()

Extract intel_cursor_check_surface() to better match the code layout
of the other plane types.

Reviewed-by: José Roberto de Souza <jose.souza@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180907152413.15761-14-ville.syrjala@linux.intel.com