Brush cursors were missing when opening a file saved in sculpt/vertex-/
weightpaint mode.

Since we dont do a full modeswitch on file load in ED_editors_init for
grease pencil since rBde994d6b7b1c, we were missing the brush cursor
toggling [ED_gpencil_toggle_brush_cursor] normally done in
ED_gpencil_setup_modes.

This is now explicitly added for any greasepencil paintmode (in case
object is active).